"It's inevitable you will loose bottom end"
Ash didn't, yet went bigger. Maybe that's how his car was setup. He DEFINATELY didn't go backwards as you're implying. Time to put the text book away and go off the guys who have spent the money trying different things AND have the numbers to prove.
Ill give you an example. My GT RS setup with a kakimoto "full mega N1" exhaust (which had a 2.5 inch restrictor in the rear muffler) was terrible. At 2.0 bar it made 414, 1.7 it made 404 or something. Got rid of that catback to a 3.5 inch and not only did it woke up the turbos but it also went alot better uptop. Then went to smaller turbos with a twin 3 inch and I made AN EXTRA 100 kw at 4000 rpm and an extra 25 ontop which is still no where near it sits today! Real world experience, real cars. Not something you have read between session on red tube!
OP. As Allan said, 34 dumps/sole sort of bellmouth jap dump, find a front pipe which is twin 75's into 90 (I think midori do a set), big cat and ATLEAST 3.5 inch cat back. Without some silly restrictor like alot of the "brand name jap" exhausts have. Having been through it a few times it really isn't that mug more to get a custom system made up which hugs the car with big meaty mufflers. If I had to do it again, that's what I would do!