Originally Posted by a5un

Trying my best to answer your questions:
1. It may help, but from my experience, the immigration is not a problem recently. You need to budget only about 5 mins to get through immigration line. Your main problem is with the transfer process itself. The worst case scenario is if you land on KLIA1 satellite -> Take bus to KLIA1 main -> clear immigration -> grab your luggage -> Take bus/train to KLIA2 -> Check in your luggage -> clear immigration again -> get to your departure gate. There're so many different factors at play here, the whole process can take from 1 hr - 2hr. At the end of the whole thing, you'll be exhausted and cursing the whole transfer experience in KUL.
2. Trains may be faster, if they depart within 5 mins of your arrival at the train/bus terminal.
3. Yes, Air Asia in KLIA2 is pretty anal about checking carry on luggage. You can purchase the check-in luggage first even if you don't end up using it. You can also carry an extra laptop bag usually.
4. Again, this may help, but in my experience 3 hrs would be enough, albeit can be a little tight.

TLDR version, 3 hrs should be enough, but the whole process would make you swear you will never do a KLIA1 -> KLIA2 transfer again.