The BG rear suspension will not be a simple direct swap to a BF. I've only heard of 2 people that have successfully completed the conversion.
The lateral arms on the BG are longer and the trailing arm's forward bushing is larger too so it won't just bolt onto the Capri's mounting point. Another issue the people that done this have mentioned is that you will need adjustable coilover setup. I'm not sure if this is due to ride height or strut/spring clearance issues.
The only performance benefit would be the one inch larger rotor. It would give you about a 15% stronger ebrake, but when it comes to regular braking it would be considerably less of an improvement (if any at all) due to the front brakes doing most of the work.
IMHO...Don't bother with BG knuckles. The build quality/reliability is superior to the BF knuckle, but handling/performance is worse. I only did the swap to see how they actually would perform if I used them for my crazy BG/GE frankenstein knuckle creation.
Now, looking at a 94 Probe G5M-R, it looks like the starter mounts right about where the stock XR2 turbo inlet would be. So the best way to make this work is to use the KL, the G5M-R, the XR2 Diff and axles, as well as BG spindles? Holy hell, and that's assuming you'd still have to get creative with plumbing as it is, not to mention the harness grafting...
If you use the 26 spline differential and the XR2 axles, you use the stock Capri knuckles.
The plumbing isn't too bad, the heater hose connect right up, but the KL's coolant exit is on the passenger side and will either need to routed under the headlight bucket or back to the firewall and ran to the driver side.
As for wiring, use the KL's charger/starter harness and the engine harness too. You only have to connect about 8 wires or so to make it run. In fact wiring in a KL setup is less work then BP setup...LOL