I think I know what's my car's problem....very easy to find out why underpower...same as lots of 4speed 1.6SX users.....
Did u all notice that 4 speed, when engine running hot, gearbox attempt shift slower and when is engine cold(cold start), it shift properly....sometimes when overtaking cars, it takes time to shift down gears, and sometimes when u accelerate, it keep on drag till for sometimes before engaging another gear....this happens when using car after few minutes when engine temperature rise to normal....
Even with stock rims, I still feel our 1.6SX Forte still powerful, but because of gear 'delay shift' which might do with gear ratio timing with accelerator not accurate....even though last week i sent my car to NAZAKIA Alor setar for checking due to underpower, they stated this problem to me, and told me they reset it back....after few days...same thing happen....I think might need install ATF cooling solution like KFC did....thinking of installing 1 in Alor Setar, but having hard time to find it, due to busy-ness.....