droning is a sound pretty much all larger exhaust will make, its when you are cruising on the highway at the same speed so the exhaust is making the same sound. Not that bad, just turn your music up
Easy to install, main difficulty is undoing 10 year old bolts and removing sensors, everything else is easy as these HKS exhaust systems are made specifically for each model skyline so no modifying necessary. I'd say a work shop would charge about $3-500 in labour alone.
The stock dump pipe, and really the entire stock exhaust, is very small and restrictive which means less air is traveling through. As for turbo lag, i dont think it would affect the stock turbo at all (atleast not noticeably)
If you are just after the sound/wank-factor, go to autobarn and pick up a canon for $50, if you want a better flowing exhaust and a bit more power, spend the $1500-$2000 and get a turbo back exhaust