Some 410 musings....
There's a small chance it was a few mph faster than that. The 388 oft-quoted has been commented on LEMB (or another forum like that, ww2aircraft.net, or something similar?) as possibly taken from a plane with external racks. Meaning the clean speed would be a few mph faster, mid 390-s. I read a short reference to this in a second place on the 'Net, but they didn't say the source, and could have been referring to the first place. More game-related I believe (one of the il2 mod pages, I think it was).
Not that I have found anything concrete. I suppose it doesn't change it much to add 7mph (12kmh quoted for racks, right?). Interesting to think about, though. Also, for comparison:
110g at 20k altitude.
100% fuel
360.........no gunpod or any ord loaded.
367 with wep
355.........gunpod only
360 with wep
347.........two rockets and gunpod
353 with wep
341.........four rockets and gunpod
348 with wep
330.........gunpod, drop tanks, and rockets
336 with wep
320.........gunpod, big bombs, drop tanks, and four rockets
324 with wep
"Clean" the 110G is close, but it is this NOE heavy role in the LWAs folks have mentioned that would see the larger performance gap, with less drag on the loadouts.
</musings>