Thanks 80shilling. That's an interesting exercise for a bit of fun. My guess is an identical 297 km/h (185 mph) for each of 1299 and 15R.
They each have the same max power (205 PS), so they will share the same Vmax if they are each geared optimally for Vmax. It's a different situation if they are undergeared (which is quote possible) and reach Vmax at an RPM that is closer to redline.
It was difficult to find a good starting reference point for this - I think everyone knows that speedos are unreliable, and there's lots of folk who've seen their bikes go to redline in 6th, but the most scientific reference I've seen is this from Motorrad on an 1199 S at the Nardo Ring.
https://www.youtube.com/watch?v=i5J6yuYDQU4
It shows GPS Vmax of 292 km/h (181 mph) at something less than redline. That means Drag = Force, and we can use the relationship that power is proportional to speed cubed. Next, check RPM from drive ratio to find engine speed was ~10,500 RPM. That's pretty close to RPM of 10,750 for Pmax (192 PS), so it seems OK to assume the P at Vmax was ~192 PS.
Then, assume that 1299/15R have the similar aero drag as the 1199 S, and assume that driveline power losses of the 1299/15R are the same as the 1199 S.
Take Pmax of 1299 and 15R at 205 PS, and assume that driveline power losses are small relative to engine power, then Vmax with optimal gearing calculates to ~297 km/h (185 mph). Check gearing of 1299 to confirm RPM of 10,700 at Vmax, which is close to RPM of 10,500 for Pmax, so it seems OK to assume the P at Vmax was ~205 PS. Check gearing of 15R to confirm RPM of 11,200 at Vmax, which is close to RPM of 11,500 for Pmax, so it seems OK to assume the P at Vmax was ~205 PS.