I'm making up my own heated gear and this is a question I've been thinking about. I'm aiming for no more than 10 amps for 1 set of gear - gloves, jacket, boot sole liners and pants topside. That's about 140 watts max for the full set.
From memory, the oem powerlet connectors are fused at 7.5 amp as you say. But I seem to remember the wire gauge into the loom isn't that great, as with the rest of the bike. It's not just the running current you have to worry about, but also the fusing current if there's a fault on the gear side.
Looking at the way those powerlet connectors are made, I wouldn't push them more than 10 amps and you will be limited to the thickness of wire you can get in them anyway. From what research I've done on commercial heated gear, they seem to be working to less than 10 amps for a 1 rider set of clobber.
My KRS came with 1 powerlet wired to the loom and the second not fitted so I was able to fit a second outlet fused direct on the battery with heavier wire.
If you want to use the oem then I would suggest adding wires in parallel - a second ground wire and a hot wire to the fuse block, then uprate the fuse to 10-15 amps.
For the second connector, I would go with an add-on and wire that via a fuse to the battery. Mine had the cutout marked on their ABS panel, just under the oem.
The powerlets have the advantage (or disadvantage?) that they can pull out if you dismount and forget. You could consider another connector solution and wire it fused direct to the battery.
Having to think about my own gear design, it worried me that the gear could drain the battery. I don't know if the Gerbins stuff comes with an affordable solution, but I'm building in a battery charge detector so my gear only powers when there are sufficient motor revs to put the battery into charge.
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