I'm not an expert, but I've done a lot of reading and I'm definitely interested in upgrading my turbos at some point too.

Generally speaking, the only turbos that are true bolt-on are the "hybrid turbos" available from multiple sources. All of these are essentially stock turbos where the compressor housing is machined to accept a bigger wheel. Nothing is done to the turbine housing, but sometimes the turbine wheels will be clipped to increase flow. The SRM and TTE units fall into this category. There are several other companies that will do the same (Blouch, Scroll, etc...)

The eliminator kit from ATP will bolt-up to the exhaust manifold, but the inlets won't. You should be able to make them fit with some custom fab work, and I've heard of several people have done similar things on the RS6. The downpipes will probably need to be modified or replaced too, but that should be pretty easy.

AMD in Seattle is building a car right now with the Tial 880's using stock manifolds, but custom inlets. I believe the 880's are similar physically to the GT28's, but I'm not 100% sure.

Personally, I'm starting with rebuilt stock turbos, gutted downpipes and a custom tune. Once that is sorted and I'm comfortable that everything is working well, I'm going to start upgrading parts like intercoolers and fuel system, then polish up my welding skills and get the GT2871r Eliminators. We shall see how far I get