Well I am not a sifu or anything but I "used" to have a perdana v6 and a cefiro a33 2.0 v6 at home, and the acceleration of the cefiro is very bad to compare with the perdana because the cefiro a33 has a heavy body and the 2.0 v6 is not sufficient enough. So it would be better for you to upgrade to a VQ25DE/VQ25DD if you dont mind another RM500 for your roadtax :)
Conversion may be costly and replacing with half cut engine also has to be carefull becouse you may not know the condition and the engine could have been tempered with oil treatment that may get the engine perform in good condition. The chances of getting a good engine is very slim. I do own a 2005 A33 and power looks very fare but very bad when climbing hill and thought of convert the engine to VQ25DE but worried on later maintenance. I am looking forward for any other inputs that may increase the engine power thru exhaust system or just a part changes without any major changes.