I'm not sure if you've addressed this or not (or maybe this is the way it already works), but I think you should grant a little more IP for TT and a little more still for Dominion. Here's my reasoning:
You get the same amount of IP for all of the following (assuming all else is equal i.e. victories and such)
2 x 60 minutes SR games
3 x 40 minute TT games
6 x 20 minute Dominion games
HOWEVER, over the course of those 6 Dominion games you will have significantly more time spent loading (about 3 times the amount) and significantly more time spent in champion select (about 3 times the amount) and the same but to a lesser extent in TT. So, your total time spent will be more in either game mode apart from SR. That takes your IP/time spent average way down on those modes.
The times for my SR and TT games may not be the average time of those games, but you get my drift don't you?
Here's a suggestion:
Maybe just grant a flat amount of like 15 extra IP for Dominion and 10 extra IP for Twisted Treeline? That way you don't have to worry about calculating load times and champ times. It would just be a flat amount for everyone.
Maybe you've already taken this into account, and if you have, would you please let us know?
FYI, I went through a couple threads that mentioned this, but I didn't really see any of them get into the details about how much time is actually spent loading and champ selecting.
Also, even if you don't like the idea, I wanted to say GREAT JOB!! You guys are doing awesome and you don't hear it enough.