I wouldn't think changing the sprockets would have any effect on whether or not you need a chain tensioner. You need to adjust your chain by changing the length of your radius rods, keeping the birdcages square by re-adjusting the torsion arm. Gearing changes should be mainly to fine-tune your drive ratio to the length of the track. Most guys I've talked to run in second. I can't help you with what you need for a 3/8 mile track; I'm still getting setup advice myself to run at Montpelier, but don't ditch the tensioner! Foz makes a pretty good setup and it's not an arm and a leg.