On my 2JZ4, I'm running 50/50 meth-water as my "cooling agent". In the 2J world, our motors can run 700-800whp no sweat for extended periods on straight 93, but you're mostly talking 1/4mile or highway pulls. Extended racing is another ballgame...
I converted my Supra to E85, and there is a large hassle factor - finding the fuel, having to fill up more often, and maintenance (can't let it sit too long without running 93 through the system, checking pump/lines often, etc).
I've stuck to 93 on the Z4 and water/meth for that factor. Meth is a lot more resilient than you think! I run the AEM 1 gal tank in the rear and the 1000cc injector (for perspective, I run two 1000cc injectors my Supra which is pushing out near 900whp on pump, which I've also run and used at Pocono raceway North course a few times!)
I've run 3 sessions at Pocono in various configurations with the 2JZ4, one of them a long IMSA course reaching over 160mph on the long straights. A full 25 minute session would run me about 1/2 gallon of water/meth. I mix Windshield washer fluid and 4 HEET bottles to get 50/50 - $1.60 per bottle, $1 for washer fluid = $8.00 rounded up Windshield wash fluid already contains 33% meth. I also run through 1/3 tank of gas (running 22lbs of boost consumes lots of fuel!). The Z's small tank may force you to have to fill up before every session
Especially if you are concerned of high corner-gs with a nearly-empty tank.
It isn't as expensive as you think, comparatively speaking. But, you get some great cooling benefits and additional safety factor built in. I also run only one nozzle pre-throttle body. How do I know the meth is working? My knock is well under thresholds during the entire two day HPDE session.
Hopefully that provides some perspective.