The barrel measures 18" so I should be good even if I screw up and have to recut the threads. I'm trying to decide if it would be better to use the parting tool to cut off about 1/4" or just cut from the crown and work my way back towards the breech. Or like you suggest, put on a muzzle device of some sort.Feels good doesn't it?
Be very careful w/ barrel length & NFA. Don't end up w/ a <16" bbl. Looks like the Mossberg 715T is advertised as 16.25". Looks like you're in the clear to trim a little but I'd double check their spec by measuring before you trim any.
You could also make a new thread protector or use a muzzle device. (If you make your own thread protector, just go buy a good tap. Toolmax (from Suncoast? on ebay) makes excellent ones.)