I always used the kit at higher rpm's over 3000 i think to avoid the detonation you speak about.

Personally I have only ever used a NX kit and as I said never had a problem with it, but as with every performance upgrade you have to make sure that you dont over do it.

The 1.6 block should handle 50hp no problem as mine did but I found that when I was testing I started with 25 (bit crap), 50 hp (crap but better) and 75 hp (goes well still need more!!!)

I think I could have probably got away with a 100 hp shot of nos but I liked my daily driver and didnt want to block the block to hell.

As with everything a careful install, and proper usage the kit will last a lifetime and will give you some fun.

On a quick note BEWARE!!! The electronic solenoids that I had as part of my kit were rated for 20 sec usage at one time as they got very hot and were prone to burning out (Not just a NX failure but accross the manufacturers). So if you are buying the kit second hand make sure they are in good working order otherwise they cost £150 a peice I think.