Health Dept Regulations in MS say "To provide a safety factor for
surges in PVC pipe using Commercial Standards the maximum pressure
should be limited to one-half of the manufacturers pressure rating of
the pipe.... For static pressure up to 80 psi, 160 psi (SDR 26) may be
used. For static pressures greater than 80 psi, 200 psi pipe (SDR 21)
should be used."

See http://www.oxfordplasticsinc.com/binder/municipal/municipal.htm.
According to this manufacturer's literature HDPE pipe is Working
Pressure Rated (WPR) and a HDPE 100psi pipe equals Class 150 PVC.

What HDPE potable water IPS pipe do I specify in order to equal PVC 80
PSI pipe? Equal 200 PSI PVC?

HDPE has better water hammer characteristics that PVC.  The shock
value is 50 % higher than the static pressure value.  So in the case
where you would spec SDR 26  you need spec DR 13.5 which will
give you a surge rating of (3/2)*128 psi or 192 psi.  You may be able
to spec DS 17 but this is equivalent to only 150 psi PVC.   Similarly
for the 200 PSI PVC spec you may be able to use DS 13.5 since the
equivalent surge ratiing is 192 psi.  However if that is not allowed
you will have to go to the DR 11 as a replacement for the SDR 21.

Good luck with your project.