well if it USED to rev to 7900rpm, and now only revs to 5900rpm, then I think that the reving to 7900rpm may have caused you some damage for sure.. I did this to a 2tg, "over-revving" and it caused some extremely big probs, basically it broke the valve guides caused by valve bounce at high rpm and then the broken bits fell into the combustion chambers and hit the pistons, hence my rebuild...
but yeah I definately be getting ya head checked....(umm the one on the car I think) *lol* sorry, didn't realise how it sounded at first...... but as for revving upto and past 8000rpm, I dun think it's a good idea.... personally I wouldn't like to be spinnin round 8000 times a minute..... would you ??
but yeah, just from what I have been told by my mechanics about over-revving.