I haven't really timed my tank/battery ratio since the first original Tamiyas (TI and M4) and using NiCads, since you had to run them down to zero to avoid memory issues. The Tiger would run full out for about 30minutes on two 1500s and the M4 about the same on one 1900. I still have those batteries in service by the way.
Since we got into IR battling a few years ago, I've switched to 3800 and 4200 NiMHs. I have two sons, so three tanks going all the time and honestly, I'd expect at least an hour and a half out of them before having to swap. If we go to an organized battle with NEAD, I usually don't have to change a battery the whole afternoon, but bigger IR battles don't necessarily mean a lot of movement.
Never went to 5000s, don't see the need, 3800s are more than enough for a good time. With a good charger, you can have a pile of batteries ready in a short time.
And in response to NiMHs drop off, yes, the lose an initial bit, but on the flip side Onyx recommends in their charger manual to exceed to the rated mAh by 10-20% for a full charge.
I use lipos when I have to (Kettenkead, Kubelwagen) but see no need otherwise. I'm in this to enjoy the hobby and the simplicity of grabbing a stack of 3800 NiMHs in a mussette bag and out the door, even if they aren't 100% charged is infinitely more enjoyable than having to be overly watchful.