I'm not that familiar with the Brewster, I've flown it twice and seen less than a dozen, but wouldn't the XVI pull away very quickly in a climb?
Co-E, Co-Alt.... The spit would be eating .50 cals before it could use its climb rate. If the Spit merged at much higher speed, then yes, he could zoom up. However, the Brewster can counter that by unloading and meeting the Spit head-on for the next merge.
Nonetheless, if they merged at similar speeds, the Spit would be eating lead before it completed its reverse. Think of the Brewster as a Zero with long ranging guns. Its turn radius is nearly 1/3 less than the Spit16's. There is no Spitfire that can out-turn a B-239. Not even close. If you get sucked into a turning contest, you will probably lose. Naturally, this is very much pilot dependent. I was in a lufbery with a Spit8 last night and was gaining ground with each turn... I was flying an A-20G. Before I could get around, someone else waxed the Spit. I believe the Spit8 is the best overall Spitfire, but a poor pilot can't utilize its capability.
My regards,
Widewing