Should I use regular .257 Roberts brass or .257+p to form cases for .257AI? Pros and or cons of each please.

Dunno. Doubt there is a difference other than the headstamp. I've been using the regular brass for my .257 AI since before there was +P

I have used the +P because that is what they had.

I have necked down 8mm brass, and that works fine too.

I tried sorting brass and doing the flash holes, but that had no effect.

The big change was when I tossed out the expander ball. Then reloads got more accurate that the first shot instead of less.

No difference in brass.

7x57 brass also works nicely and allows you to set your neck/shoulder datum point to exactly fit your chamber on the first shot. It will also give you a slightly thicker neck that is likely to fit your chamber better.

The difference in standard and "+P" brass is the headstamp.

I'd have to second Stonecreek's 7X57 recomendation as the best I have used in my 257 AI was some RWS 7 x 57. Little thicker necks when you are done, fit my chamber better.

To get 257 brass, I neck down the 8x57 in a 7x57 Wilson vise die.
Then I neck it down again in a 257 Roberts Wilson vise die.
I have never had a problem with the neck being too thick with a standard 257RAI chamber.

Been using regular 257 brass in mine, works great. I haven't tried 7 Mauser yet, it should work fine though.