Stage one would be a remap of the 2.5L for a little more alert feel. Close to 200hp
stage two would be 3.0L inlet runners and baseplate (or porting your 2.5L baseplates to match the chrome runners.) With remap. Just over 200hp and a free reving 2.5L whuch really is fun to drive and quite low on gas consumption.
stage three would be like above but with GTA camshafts, larger plenum, 215-220hp or so.
However the lack of torque will be substantial compared to a 3.0 which delivers a whole lotta more from 2000rpm to redline.
I do love the 3.0 with its power in the 166 but the 2.5 156 with a few mods really is a lively little six with a truckload of character that loves to be driven at high revs. I suppose a lot of that has to do with the 2.5 gearing though which makes you smile all day long.
'77 Giugiaro GTV 2.0: newly restored, fresh Rosso 530 paint/tan custom gtv6 interior, ported&skimmed, CB47, high compr.pistons, iresa header: 160hp
'06 Bertone GT 3.2: metalic black/red interior, lowered, custom alu adapter and 78mm Porsche throttle body.