Are you saying you are not geting good groups with 350 gr. or 400 gr. bullets? Or are you getting good groups but not on target?
Changing bullet weights will change your point of impact, and yes, you need to change your sight seting to compensate for the difference.
I had the same problem with the .44 Magnum using 180 gr. and 240 gr. bullets.
I solved the problem by buying two different revolvers and sighting them for the different bullets. Then bought different grips to tell them apart.