I know I am a new guy here, but I may be able to add a little.
I have owned a 670h.o., and they are fast little sleds, but your 800 (have a 2001 zl800) should walk past the 670.
Mods for the 670 h.o. are hard to come by because they are modded from the factory about as heavily as they can for a production sled.
My 670 would run side by side with a buddies mxz700, but my zl 800 walks the 700.
In my opinion the goodwin kit is a mutch better ballanced kit than the d&d, and their technical support is far better.
been doing the round and round with d&d with my pipe and chip combo on my zr600, finally got it running right, and the parts kick a$$, but their support sucks.
p.s. here is the setup I am using on my 800.
Primary purple spring
Secondary red / white spring (middle hole)
73 gram weights
60 / 52 helix
Stock gearing (2002 stock set-up with helix change)
Glad to find this site,