I was referring to sea level.... I have no idea how you concluded the old Mossie was faster down low.
The new Mosquito is faster at all altitudes as compared to the old Mosquito when on WEP. At MIL the speeds are very similar, with the old one perhaps edging it a by 1mph at sea level, the old one faster by an odd 13mph at 5,000ft and the new one 4mph faster at 12,000ft and 1mph faster at 13,000ft.
100ft:
MIL: New: 319mph Old: 320mph
WEP: New: 357mph Old: 338mph
5,000ft:
MIL: New: 334mph Old: 347mph
WEP: New: 374mph Old: 363mph
12,000ft:
MIL: New: 361mph Old: 357mph
WEP: New: 384mph Old: 376mph
13,000ft:
MIL: New: 363mph Old: 362mph
WEP: New: 386mph Old: 380mph
I printed off a Mossi speed chart from the AH Trainers website, that still has the old Mossi stats (old Mossi weight is 2000lbs heavier as well!!!). I then placed over top the new Mossi speed chart and I'm seeing a bit bigger of a spread than the stats you posted, my hunch was correct. For instance;
100ft:
New: 318mph, Old: 322mph [difference of 4, yours had a difference of 1]
5000ft:
New: 335mph, Old: 348mph [a difference of 13, same for both]
10,000ft:
New:352mph, Old: 364mph [a difference of 12, yours had a difference of 4 in favor of the new Mossi???]
13,000:
New: 363mph, Old: 362mph [difference of 1, same for both]
15,000:
New: 355mph, Old: 372mph [18mph faster for old!)
At 19k, the new Mossi finally and clearly over takes the old Mossi for a higher top speed and they don't become real closely similar again until about 30k when they both move at 340mph or so.
Aside from 12k to 13k or so, and above 19k, the old Mossi was clearly faster. Map out a chart and see for yourself. Obviously, I'm not even debating WEP, there is no confusion on that.