That is about what I would expect.
Also, from personal experience running a 14v setup(16.8 charge) I would not recommend running a higher voltage just to get more power. If you want more power, buy the bigger amp, it will save you money in the long run.
The reason that I say this is that the d1400 and d1600 batteries have about half of the reserve capacity of the same size 12v battery(d3400). If I had it to do over again, I would just get the D3100s and call it a day. Unless you are planning on just burping and only need that 3 second burst, I would go for the D3100s for any daily application. They have a much higher reserve capacity and you can beat on them much longer before needing to recharge. Also the D1600s will require a much larger battery bank.
Taking alternator charging capabilities out of the equation, here are my recommendations for battery power.
5k vs 7.5k
5k on 16v, I would run 6-7 d1600s- $269.99 shipped each ($1889.93)
7.5k on 12v, I would run 4 D3100s- $369.99 shipped each($1479.96)
The thing is 4ohm rating so the 7.5k is out of the question. The plan is running two eventually. The 7.5k would put me in the higher class where two 5ks won't.
In super stock and mod you can run up to 18v so that's why I'm considering it. When running the lanes I'd have to dial the alt back pretty far to run a 16v setup. Will 5ks operate in the 19.x volt range?
http://www.soundoff.org/rules.htm
So I have to stay under 1800 @4ohms which is based on 14.4v published ratings.
Granted I'd be limited to 6 batts for burps. I'm tossing around ideas of a super cap setup too.