I did my first track day (at Wakefield park.. its a smaller circuit so NA's are not as disadvantaged) a couple of weeks ago with very similar mods as you:
HSD coilovers
Front and rear strut bars
Heavy duty clutch
Full extractor back exhaust system
Drivers fixed back bucket
Removed front seat, beack seat, gutted boot
GTR bonnet
And a pretty sweet set of semi slicks (khumo V700) on R33 GTST rims
Stock brakes with high temp brke fluid (I have a GTS4 so its got 4 turbo brakes front and rear)
In the straights I was loosing ALOT of time!
In the corners though I was passing (and being held up by) cars like HSV's, SS's, and even the odd GTR and evo. The car relly really flew through the corners, it held its ground without any problems
The best peices of advise I can give you that I notied worked best are:
Its worth the money to invest in good rubber! Some people were using street tyres. The level of grip on semi's is just MINDBLOWING! I cannot stress this enough! To be quick on the track these are essential!
Brake as hard and in as short of a distance as possible. I noticed while attacking other people on the track that many people use regukar street driving braking techniques. The way that I found best was resist the first time you get the urge to brake, resist the second time and when you hit the "oh shit!" button, jump on the anchors, HARD!
This being said - I boild my brake fluid and the brake went flat to the firewall (being th smartarse I am.. I thought.. "its gotta be a quick lap if I cant use my brakes!.. it was ) So high temp fluid is an absolute must!
Thats my 2cents
I hope it helps