The two mods known to cause significant startup delay with GT are Binnie's Mods and RotaryCraft. There are rumors that Extra Utilities also increases load time notably, but I cannot confirm them because I haven't tested that.
As for other things, you could try adding Player's FastCraft to your pack, as it has been known to improve performance in general.
I tried removing ExtraUtilities and the startup went from 120 seconds to 110, so nothing major (like from 340 to 120 after removing Binnie).
I always use FastCraft in 1.7 though.
Thanks for the tips.