That number isn't low...I've had 2 Z/28s, a '15 black and my current is a '14 red.
My current runs the same cam (GT19) and heads from LPE that you have, and I have kooks 1 7/8" long tubes, with no intake, and I have a soler ported TB (to help with the lag/drag on initial takeoff). Prior to the TB, mine dynoed at 545/489.4...the TB didn't add much of anything, but *maybe* puts it around 550 (but you really only feel the difference when taking off with some gas pedal applied). The FAST intake, which I had on my last Z/28, is probably giving you that extra 12whp. Personally I would advise AGAINST doing the 102mm TB. I did that with my last Z/28 and had issues getting a tune right where the car wouldn't stall at low RPM (read that as "backing out of the garage" RPMs) either going forward or backward. I had a tuner work around it, but every cold start was a "soot shoot" (my garage is STILL stained from it, 3 yrs later). I had multiple tuners tell me it was because of the 102mm TB I put on (and went on to say to stick with the stock, or maybe a ported stock, until around 600whp).
Now, whether or not thats true, and to be fair I have had a couple knowledgeable folks say it isn't, I had 3 different tuners not associated with each other who actually worked on my car tell me the same exact thing, at different times.
Wait, was there an actual question here, or are we just comparing things?
I might have the spec sheets for the LPE heads and GT19 cam still, I think I do....so if thats what you're looking for, I might be able to help
I will say that I called LPE to find out if they used the bronze guides, and come to find out, they are using PM just like stock (a beefed up variation iirc, but still PM).