cool........
well, fitting a larget TB aint as simple as that. But essentially, yes, you could do that, but it wont give the gains of a full set of throttle bodies. So, its roughly 700 quid for the larger TB, then remap and RR time so well say about 1200 quid. OR the full weber alpha gold kit for 2K and a lot more pwer and tractability. But, cost might be an issue.
BHP increase i cant say, as itll be the sum total of the TB gains, plus the remap and operators skills. But im not sure if many places have the ability to just "remap" the RENIX system as its quite a fortress. Thats why the whole new replacement management system might be abetter choice as you can control more parameters and more people are experienced with the software.