It's normal to feel/hear clutch slippage during "spirited acceleration" gear shifts into higher gears. In low gear, the engine can barely free rev as fast as the bike will accelerate - even if it is slipping a bit you would be hard pressed to hear it. In the higher gears full throttle will be pulling against the steep gearing and clutch slip is both worse and more noticeable.
A slipping transmission has a very brutal sound and feel. You can't have that and not think your gearbox is self destructing (it is).
Replace the clutch friction disks and deglaze the steels. If the springs are under the free length service limit, replace them. Do not go to heavy springs with a stock engine, they are not necessary and the extra level pull will not be fun.
There are many post here about clutch rebuilding. Make sure the steels go back the same way around (they should all cup the same way and they are never perfectly flat), soak the friction disks in motor oil before assembly, and remove any notches in the basket finger edges that the plates could hang up on - just file them flat and do about the same amount of filing on all fingers. a bit oversize doesn't affect operation.
