Pm Hw Extreme....
the other difference in the engine's it the exhaust cams close the 2 valve's at slightly different times to try to smooth the engine.
the different plemuim is to add torque.
Easiest & cheapest mods are air intake, ECU, Exhaust, 98Ron.maybe a power boost valve(if they are any good, Anyone know?) you can go mad though. Balance the engine, port & polish the heads, fit 1830c.c piston's & of course lighten it.
But before you make any power mods please make your car stop better. Also a Good handling car will be quicker than a more powerful bad handling car. If you can use more power more of the time, your gonna be faster.
2x 145, 4x 75, 4x Sprints,a 156, 146 now a 155