My warrior is currently level 30 and I just got slam. The main goal is to kill a single enemy as fast as possible.

Currently I am considering speccing improved slam, but I would like to hear some opinions.

Currently I am using a 2.7s speed main hand and a fast offhand weapon.

While using a 1.5s weapon as a mainhand, slam seems pretty weak. It disables main and offhand for 1.5s and delivers mainhand damage + x, where x is lower than the heroic strike bonus.

But using my 2.7s speed weapon slam delivers 2*weapon damage+x, within 1 mainhand cycle.
Is this worth giving up
a) offhand weapon damage and rage
b) main hand damage + heroic strike bonus

Slam is for two handers, it's not normalised because it has a cast time so you get a bigger AP contribution from having a slow ass weapon. Stick to Heroic Strike because at least that way your offhand is still swinging while it's queued.

Why don't you pick up your Whirlwind Axe and play with Slam that way?

[edit]
And where do you get the 2*weapon damage thing from? Slam is (ap/14 * Weapon Speed) + Average Weapon Damage + Slam Bonus Damage

This has been covered in-depth on the WoW forums.

I've recently taken improved slam with my 65 warrior on a trial basis. The cast time is now less than the global cooldown, which is OK for fights where you want to cast it and move quickly. While leveling, you're not going to have to move around too much and therefore I think the points are wasted by bringing cast time under the GCD.

This is even more relevant at level 30 since you have more than half of your leveling ahead of you.

Slam resets your autoswing timer after completing its cast, which basically means that Improved Slam makes your next autoswing after slamming come 1 second earlier. This is very significant for the dps boost from the ability, even when you don't need to move immediately. It also means that it's absolutely crucial that you always Slam immediately after an autoattack and never at any other time, since that leads to an unacceptable loss in white damage and overall dps.

Slam is completely unusable crap with a DW setup, so don't bother if you haven't gotten a whirlwind weapon yet.