This is a common theme that frequently shows up. While Rawr.Mage gives an answer to this in the Item Budget graph, I think this is not the right way to look at upgrades.
There are essentially two ways how to handle upgrades. One way is to look at all available gear, create the ultimate set and then work your way towards it. Another approach is to look at upgrades in a progressive manner, looking at which items make the greatest improvement over your current item set. I think ultimately the best way is to combine both of these and I'll provide some information about how you can use Rawr to search for progressive upgrades.
There is an important problem that one can overlook when search for upgrades. This is the problem of looking at one particular fight setting and trying to make generalizations to different fight conditions. The way I'm using Rawr is I have rawr character profiles for different fight conditions that are common in raiding. Different fight lengths, different combinations of raid buffs, special fights including resists and similar. You don't need to cover every possible situation, just what is common in your everyday raiding.
After you have all fight conditions set up you should mark all items that you have available (i.e. in your inventory). I would suggest that you mark only the specific gemming that you're using currently (you do this with control-clicking the diamond so that it turns blue as opposed to green) and select the current enchant on the item (right click the diamond and select the enchant). This will be helpful for progressive upgrades unless you plan to regem everything whenever you get a new item. You can still look at how to best regem your gear when you set it up this way by checking Override Regem option in the optimizer.
Now the questions is, given these fight conditions and current available items, how do you determine which gear will give you the best upgrade overall over all fight conditions.
To answer this question you have the Batch Tools available from the Tools menu. First thing you should do is load all fight settings you created. You can either do File->Import and select all character profile files or add them individually by clicking on the ... button. You can save the set of fight conditions with File->Save so that you can reuse it later.
This should give you a list of characters for different fight conditions with a score for each. You will see a column called Weight. You can use this to adjust relative importance of different fight conditions. For example if you don't want the resist fight have much importance, but still take it into account, you lower its weight. You can quickly switch between different profiles by clicking on the Show button.
Let's look at the tools that are available. First we have Set Available Items. This will copy the items marked available in the currently open character to all profilese in the batch list. So you don't have to make changes in each profile, you just make it in one of them and then set it for all.
The main tools are Optimize and Build Upgrade List. Optimize is the equivalent of optimizing each individual profile. When you do this any profile that finds an improvement will be marked with a * and a value of new score will be displayed in its column. After optimization is complete you can either save all new profiles with Save Characters or use Save Characters as Copy to leave the current profiles intact and save the optimized ones in a separate xml file.
Finally let's look at what we initially set out to do. For build upgrade list I suggest setting thoroughness to minimum and disabling regem/reenchant overrides. The batch version of Build Upgrade List is distinct in the way it evaluates gear. It won't create separate upgrade lists for each fight condition, instead it will look at upgrades and see how much of an upgrade it provides for each of them and compute a weighted average using the specified weight.
What this means is that even though for a very long fight it might be good to think about mana regen, it will be balanced against other fight conditions so on a overall very good item it will use gems that are good overall. At the same time it will be able to point out potential sidegrades. If one item is very good in a specific fight condition, it will realize that and suggest gemming and enchant that is in line with that. You can also use this to find a new purpose for items that you replaced. For example they might fit well to fill the niche when you have to gear for when you don't have draenei aura in the group. Similarly look at items that you have available as there might be a gemming option available that is better overall than what you're currently using.
Last just a note on how to keep such a setup updated once you get new gear. First decide how to gem and enchant the gear and then make appropriate changes in which items you have marked as available. If you regemmed/reenchanted some item that you were using in some of the profiles you can use the Replace Unavailable option. This will look through your profiles and look for items that are no longer marked as available and replace them with a gemming combination that is now available. Finally you run the optimize and update your ingame item sets.
Indeed the 2.1.4 data files have a Cincture with a gem. This can happen when you do load from armory on a character with a Belt Buckle.
Coincidentally, there is a gnome mage in Astrylian's guild with Cincture and a red gem
Honestly, this is one of the huge reasons I keep an entirely separate ItemCache.xml file to swap into each version. It almost feels like the cache could use it's own maintainer at this point as it's gotten to the point where the product is almost unusable out of the box.