You could bring a TSI to GTI levels fairly easily yes, but say kiss your warranty good bye. GTI's come with all those "mods" and [mostly] everything is covered by VW.
I know what you mean btw, but I fully understand why some people buy GTI's instead of modding TSI's.
There's much more than the engine, the turbo and the springs. The differential is much better, the sway bars are bigger iirc, etc. It's a totally different beast.