I have a nasty toyosports full system fitted to my 03 blob sti, I have a Prodrive back box that wont fit, so my questions are

1, what is the limit of the Prodrive box, does it flow good so I only have to get a down pipe and centre section.
2, should I get a full system, cobra ones look ok would this be better

 

1. Prodrive box is good. Looks nice and oem, makes a subtle noise (but still a cool noise on boost), and as a 2.5" bore will flow plenty enough up until the high 300's bhp. (maybe even more).

2. Down to your preference really. The 2.5" bore prodrive box will mate up to any 2.5" bore system. Depends on whether you can get a full turbo back system for a good price.

I have a 3" decat downpipe on my widetrack STI which reduces down to a 2.5". Then I have the standard centre section with silencer, and then a prodrive rear box. It flows great (remapped to 330bhp at the min), is quiet when cruising round. But howls on boost

 

It sounds like your Toyosport exhaust is 3" bore, you will need a 2.5" bore exhaust to mate up to your Prodrive back box.

My downpipe is a 3" all the way though, and mates up to a 3" powerflow cat bcak system.