I generally consider about 18% drivetrain losses for an automatic, but completely agree that dyno numbers can be misleading based on a number of factors.
Still - yes, that seems low to me. You didn't note if the AFRs were Streets or Comps, but I assumed streets. I assume the cam is the XE284H? And finally - what RPM were the numbers achived at?
My interest centers on the cam - it's defnitely a high HP cam; I did a quick DD Advanced of your combo and reached 450 FWHP @ 6500 RPM. I haven't done the quarter mile math but based on your gearing it would seem that's in the ballpark. Those heads kind of demand a roller hydraulic, or roller solid IMHO.
I can throw some more research into the pot, but my quick take is a) I agree - take it to the strip
and b) not 100% confident that cam choice is a good match for the heads.