This has been a work in progress for a while... and I finally had a chance to test them.
As an alternative to the AP calipers in the rear, I decided to try a pair of HiSpec Ultralite2 calipers. They have several piston options - I elected for the 38.6mm pistons. The calipers are light... more-so than the AP calipers. I don't have a weight, sorry.
To mount them, I used a pair of modified EliseParts brackets. ~2.5mm spacers were required between the bracket & knuckle. 4.5mm spacers were needed to achieve the proper radial spacing. After receiving them with pads I ordered from HiSpec, the pads are nearly identical in shape to the 2-piston AP pads, i.e., it's feasible to run the same pads front and rear. You'll need to remove a small amount of material (2.5mm) from under the heads of the hex head cap screws to keep them from hitting the axles due to the 2.5mm spacers between the bracket and knuckle.
Brake lines: I used a male-male M10x1.0 "metric convex seat" to -3AN adapter at the chassis connection. I used a -3AN Wilwood hose from the inboard attachment to a -3AN to M10 banjo fitting adapter (from Pegasusautoracing.com) and an M10x1.0 banjo bolt (also from Pegasus) with copper washers on each side of the banjo fitting.
I elected to remove the stock calipers and forgo parking brakes. I previously had an S2000 with a similar setup, so I'm use to not having a parking brake.
The good: cheap caliper option, able to use the same pads front and rear, minor modifications requires with existing caliper adapter brackets (BOE or EP), relatively simple hose adapters, good piston size option
The bad: here's where it gets a little iffy... I have one caliper that doesn't want to release after braking force is applied. This resulted in the caliper temperature being ~100F higher than the opposite side. It makes an annoying sound due to running track pads and will ultimately result in increased wear and slower lap times due to constant friction being applied. I need to dissect the caliper to investigate why this is happening. HiSpec doesn't have the best reviews... maybe this is why. I'm already working on a more elegant non-AP option using different calipers. More to come on that front, though I work pretty slowly these days.
Pictures... because that's what everyone wants to see...
AP 2-piston pad on left, HiSpec pad on right:
Modified EP bracket:
2.5mm spacer between bracket & knuckle:
4.5mm spacer between bracket & caliper:
Caliper spacing relative to rotor:
Inboard hose adapter:
Outboard hose attachment: