I used Mazda E3 pistons. They are cheap, and flat top. Unsure of weight compared to A15 piston.
In my engine with block and head planed to get comp ratio up, we had to mill valve cutouts as the cam has quite high lift. Was getting pretting marginal (depth of cutout) so if you're motor's really wild keep that in mind.
It was a while ago and I looked at other options, like Daihatsu, but they come in sets of 3, so you need 2 sets. Also you said $309 for pistons then more for rings? Unusual for a set of pistons to come without rings isn't it?
With the mazdas they come in up to 2mm oversize, so with their std bore being 77mm you can build a 79mm A15.
I saw somewhere (may have been feral's site- not sure...) sets of forged 79mm pistons with a big bore head gskt for A$900 I think?? Doesn't sound too bad, that's probably the road I'd go down if I were to do it again... but that's just my 2c!