Theoretically, if you're making bales with 15 flakes per bale, a plunger speed of 105 strikes per minute will give you 7 bales a minute, which is 420 per hour. That's obviously best case - ideal windrows, no turns allowed
I think most of the high capacity NH offset balers are 90 stroke max?, and I think the 315 is 79? That would yield a max of about 315 bales per hour, so all things considered looks like you're managing about 87% of max, applied to a 90 stroke machine that would only push you up into the 320 bales per hour range.
The newer 105 stroke machines (which I understand you don't want to spend the money on) would be at about 365/hour (at 87%)
That's all back of the napkin stuff, obviously. But it should be pretty close - I just baled 190 bales in a very less than ideal field yesterday with my 311 (70 strokes per minute) in about 50 minutes...
Theory is great, reality can differ. Many guys using bale wagons that count strokes to ensure bale length consistency target 12 strokes per bale. 15 strokes per bale isn't going to provide much throughput. While fine tuning knotters I have observed 570 and 575 operators banging out 36" straw bales in 7 or 8 strokes. That makes a bunch of bales in a hurry.