I'll do about 6 runs on a fairly long track (they are usually in the neighborhood of 60 seconds) with lots of high speed WOT sections and generally I won't use up more than 1/8 tank or so. In fact if I fill it up, I leave the track with the needle still barely on Full.
However most of the time I run a 1/2 tank or less. I don't really notice the sloshing and I don't want to starve the fuel pump by running it empty. I do run a fairly stiff suspension though so that might have a little to do with it. I seem to be much faster without a ton of gas in the tank since my car has 0 power and likes to maintain low RPM (auto).
If you know where your weight is shifting then the sloshing isn't too much of an issue IMO. The only time it's really funky is quick turns like slaloms or chicago boxes and if you're pretty fluid with those generally you can keep it all together. With long sweepers you know where your weight is going and you can still transition smoothly.
Yes, that is a banana in my pocket.
Dan - gump
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Wiregrass Region SCCA