Fastest Phantom (that i know of): 4-5 BRs
Fastest Venom (ditto): 8 BRs

For a given amount of airtime at venom speed you wouldn't travel as far as you would at phantom speed if you'd stayed on the track so the boost gained relative to the distance that would otherwise have been covered is much greater at venom speed.
something like that...making vid now - will post here when it's uploaded.