Thanks for the info.
It is also possible to use Denso "U-groove" K16R-U11 plugs. I double checked with Denso USA. These are listed specifically for the DOHC 2.5RS and cross-reference with PFR5B-11. Excellent copper plugs, about $1.49 each mail order.
I ordered mine from www.importpartsspec.com.
BTW, they also carry Nippon (the old Subaru OEM, pre-Purolator) air and oil filters. They have a screwy online catalog and sent me the wrong stuff. I called and they sent me the right stuff, no problem. So I would recommend you call them and ask for specific parts (i.e., make clear you want Nippon, not the other brand) and use the online catalog only as a guide.