This is going to be an un-scientific comparison, but its as good as I think it can get it.
The very first thread in the Engine, Exhaust, and Transmission sub-forum is on the general calculated differences between Mustang, Dynojet, and Dynapack dynos.
According to that, the community *should* understand as a general reference that
Dynapacks are marked as reading 8-10% higher than Dynojets. The numbers you posted are 374.382 on the Dynapack and 328.55 on the Dynojet. If we run a 10% difference for your numbers between dynos, then:
374.382 Dynapack * .9 Dynojet correcter =
336.9438whp Dynojet corrected
Its hard to believe that a 8 hp difference would even remotely be considered as so far off that it's head is "in the clouds" given your stated number of 328.55 and the general rule of thumb, differences in weather between the two days, and whether this is a worst run to best run comparison.
To regularly expect 320-340whp on a Dynojet seems
extremely conservative. Personal experience with a C38 and E85 tells me its higher, especially with supporting mods.
A while back, I put down 330whp with a C38 on a Mustang dyno. There are folks with C38's in the KW Owner's thread that have made more than that (albeit on Dynapacks, but when corrected, they were still higher). Again, using the general rule of thumb that Mustangs read 12% lower than Dynojets, then correcting for that: (nevermind that where I did it they claim its a 17% difference compared to other local Dynojets)
330 Mustang * 1.12 Dynojet correcter =
C38 369.6whp Dynojet corrected (rule of thumb)
330 Mustang * 1.17 Dynojet correcter =
C38 386.1whp Dynojet corrected (their stated difference based on local Dynojet comparisons)
That was with the stock headers, overpipe, and front pipe, and not a lot of tuning time.
I'm currently at 370whp from the Dynapack. Again, following the rule of thumb:
370 Dynapack * .9 Dynojet correcter =
C30 HBP 333whp Dynojet corrected
386 (C38 old setup, Dynojet corrected) - 333 (C30 HBP new setup, Dynojet corrected) =
53 hp difference, not maxed out
Driving between the two cars, that difference seems to be about right. That makes going back to a C38 enticing, especially given that neither setup was actually set up to achieve their full potential. I still have a bit of room left on the C30 with the high boost pulley, IMO. I believe I might be able to hit the 340-350whp Dynojet mark (and higher on the Dynapack) with a different header and overpipe setup, a fuel pump change, and some additional tuning time.
TL;DR
Only being able to hit 320-340 on a regular dyno (seems to be a Dynojet dyno) with a C38 seems factually incorrect, and the "Miracle" dyno does not have "its head in the clouds." The publicly posted dyno numbers from community members and general, readily available information on the differences calculated between dyno types in the community/this forum seem to corroborate that. I might do some back-to-back dyno testing to see some differences for myself after my next tuning appointment.