as far as I've heard from those who are alot faster than me and have been doing this for a long time, with a turbo there is no need to have any additional back pressure, as the exhaust wheel produces enough.
For normally aspirated engines, yes, you need some unless you have a full race motor. So alot of exhaust shops still go by that, and also they do TONS more work on V8 guys etc.
The 3" will be more than you need on the 13g, but it won't hurt anything. It might cause boost creep tho, you do have to watch out for that unless you have some provisions made for that?
All that said, unless you are planning on making tons of power down the road, a 2.5" exhaust will not be a restriction. Lowell went 12.28 on an Evo 16g, 2.5" exhaust and some minor fuel upgrades.
As for 3" mufflers, I have a 3" Borla, and I've seen plenty of 3" Magnaflows. That shop saying that you can't get one kinda shoots thier credibility in the foot.