Generally speaking, wouldn't this issue only crop up for DoTs and in that case it only is important that the effect of an item like [Heart of Ignacious] is active as you proc Eclipse and cast your next round of DoTs (therefore getting the effect of the item proc over the full duration of your DoT).

So ultimately, it would benefit only you to delay the use of the item up until the point where it's effect lasts until you reapply your DoTs, correct? Again it's something of a fringe effect that is going to be hard to judge, especially considering that all it would take is a few procs from Euphoria to throw off your rotation's timing.

Right, because Haste only directly increases damage of DoTs. For everything else, it only increases the rate at which you do damage. This is different from Spellpower/Int, which increases directly increases the damage of all your spells, and therefore is subject to the fluctuations of percentage based buffs like Eclipse.

On the topic of [Heart of Ignacious] specifically, here is what I had come up with:
The added damage from delaying the proc is going to have to outweigh the damage "lost" by not proccing the effect (which is a product of the reduction in uptime of the buff). A full stack would net you 1815 Haste, which is currently valued at right around 3085 DPS. The stacking Spellpower effect would total up to 435 spellpower, which nets you right around 1085. So in order for you to have a net-gain, you would have to pick up an extra 2000 damage for every second you wait to use the proc. And again, this damage is only going to come from your DoTs.

From what I've figured using WrathCalcs, the difference in damage between Insect Swarm and Sunfire (assuming a Solar Eclipse) with the proc and without it is ~26500 damage which would mean you could conceivably delay the proc by 13 seconds before it would be better off simply using the proc.

That's an incredibly large window honestly and it sparked another thought... Isn't the same also true for delaying an application of a DoT when the cooldown for the use effect is close to expiring?

Editted Note: Obviously, I'm using the DPS values for Haste and Spellpower from WrathCalcs, which are going to fluctuate slightly depending on your exact gear make up. The same is true for the damage output of Insect Swarm and Sunfire. Lower gear levels will make that "13 second" window smaller, because the DoTs will do less damage.

Does anyone know if Euphoria works like the attack table or if it's merely a % proc when gaining energy? I know that when hitcapped this isn't a issue, since both the table and the proc would behave samely, but in the case you're not hitcapped, it could lead to slightly different results:
If you are 1% under hitcap:
If it behaves like a table, you have 1% chances of missing, 24% chances of proccing double energy and 75% chances proccing normal energy when casting a spell.
If it's a proc when gaining energy, you have 1% chances of missing, 23.76% chances of proccing double energy and 75.24% chances of proccing normal energy when casting a spell.

Yeah, I know the differences are trivial, but maybe at higher tiers (if haste > hit) this would be useful, so I wanted to share my thoughts.

nvm delete

Rogues, at least, have found their level 85 trinkets to have a 75 second ICD, so I don't think 45 seconds is a safe assumption.