I have no experience of HP5+ in sheet form but given that it is HP5+ and only expired in 2006 I'd be surprised if you don't get box speed with Microphen as it's a speed increasing developer, assuming that what you want to use.

Wait to hear what others have to say about this combo but as 8x10 sheets are not stuff to be wasted I wouldn't drop E.I. below 250 maybe even 320 unless those experienced in this combo definitely say to do so.