Do the above only damage the clutch or do they harm the gearbox and other things as well?

Whats a good launching technique for cars with around 300hp and stock clutch? what about rollout?

http://www.ntptransmit.co.uk/peccy/launch6000.wmv

above clip of my old turbo 2000 running 280bhp and AP organic clutch doing a high (6000+) rpm launch. best way to lauch, BUT in a year i went through 2 or 3 clutches and flywheels and 2 gearboxes, although the UK box's are made from chocolate.

you have probably done one too many hard launches. do you drag race a lot?

I've done approx 12 high rpm launches in MY03 WRX for a year. the gearbox seems to have gotten a bit crunchier and sometimes when i am redlining in second I have trouble slotting into third. does it sound like a clutch issue?

Someone adviced me to change gear oil. dunno about that somehow I doubt its gonna help

With 300 bhp on a stock clutch you need to feed power gradually whenever possible.

If you bear in mind that the clutch is there to protect the rest of the drivetrain you won't go far wrong.

I ran an STiRA on track for nearly two years (25+ trackdays) on a standard UK clutch, only slipped if I side-stepped- which I avoided like the plague. No slippage whatsoever under normal circumstances.

The problems you describe on your MY03 sound synchro related. Maybe try a high quality gearbox oil like Redline 79w90.

I hope this helps;)