Talked with Matt at GiroDisc, who was REALLY nice and helpful.
He referred me to Zeckhausen racing, who knows and does a lot with Brembo rebuild parts. Spoke with Zeckhausen and got this great info:
A) Piston / seal / dust boot sizes should be cross-compatible between Brembo Subaru OEM and Brembo GT.
B) Stoptech seals and dust boot kits are cross-compatible with Brembo calipers.
C) Stoptech pistons are generally cross-compatible with Brembo calipers, though the Brembo pistons should be measured first to confirm.
D) Stoptech pistons run about $25 each, so not the end of the world if you find a piston is not true.
Separately, spoke with Matt about powdercoating the calipers.
He said that though it's a matter of preference, he actually recommends powdercoating. He also said the obvious, that even if I do get them powdercoated, they'll just get messed up again through long-term use. He said he has seen very rare instances where a caliper will deform during the powdercoating process. He said I should also consider sanding them down and then painting them with a high-temp paint and a high temp clear. I like this idea, since it will save me some money
Matt was not at all pushy. He was just a nice guy who was very helpful and was kind enough to share his personal experience with me. Disclaimer: this was coming from him, not from GiroDisc as an official policy.