Is there any rear caliper with e-brake & >38mm piston for 914/6GT ? Options

[amallagh]

My 914/6GT is struggling a bit with brakes having uprated to a 2.8 twin plug with about 280bhp.
It currently has S calipers up front and some 914/6GT rears with Porterfield R4 pads.
As well as heat the sticky track tyre set up could cope with more brake torque as well. (215/50r16 front and 245/45r16 rear). It is not a single use car - It is used on road as well as competitive rally and some trackdays.

There are plenty of 911 uprate options for the front, but without changing to a 911 emergency brake on the rear, then I haven't been able to find something to balance things at the rear. The current set up is properly balanced and there is no point in uprating the front unless I can uprate the rear. I really don't want to mess about with the rear arms by fitting the 911 e-brake system unless I really have to.
Is there any alternative rear caliper out there with a cable operated handbrake mechanism, with a >38mm piston like the original 914/6 calipers ? Anything from an Audi or VW for example that anyone knows will work in this application ?
Something with a 42 mm piston (like the Carrera rears) or even slightly larger would be ideal. Or has anyone fitted larger discs to achieve a similar upgrade ? If I could uprate the rears, then I could fit some Boxster front calipers on 280mm Carrera discs or a Boxster caliper kit with larger 300mm discs which will still fit under my 16" wheels which I definitely want to keep.

Am I wasting my time looking for an uprate option for the rears which still uses the 914/6 style cable operated handbrake in the calipers.

The crucial things is not having to change everything over to a 911 style e-brake set up. Any advice/options very welcome.
Regards
Andrew