My car was just about at the upper limit of the STI7/8 topmount at 370bhp-ish, it was also at the limit of the standard STI pink injectors. As you know I now have a big APS front mount and water/methanol injections. But am still limited by the injectors.
A top mount will always give better drivability than a front mount, but you are obviously limited by power as a result. You also get heat soak in traffic.
However, the 'lower the spec' turbo and the more you push it, the higher the intake charges will be as a result. So a front nmount will help unles you are running a high flow hybrid turbo.
Until recently, FMIC's were all around or over £1000, so people struggled on with top mounts due to budgets. Howerer, there are a lot of cheap ones out there now for the classic, so it actually makes sense to go that route.
Waterinjection actually richens up the fuelling which will loose you power. Hence you need to lean it off at the relevant points. It was this scope that allowed me to get a bit more out of the pinks.