Lol. I didn't want to appear to be a buzzkiller by pointing it out at the time but I kinda figured you'd have some brake issues. You've pulled of a really nice swap. I've seen a couple others but only when they were up for sale. I assume the brakes were one of the reasons they may not have worked as well as expected. Nice to see someone with some follow through and commitment building a monster like yours..
Yes, the brake is already a bit undersized for the TW. I cover and use both brakes no matter what I'm riding, but with so much rubber on the ground you'll use it harder and more often than you would on something comparable like an XT with the same sized brake. I don't think fade even enters into it on our little bikes, it's more like the fact that not enough happens quite quickly enough when you really need it. .
Offroad is where I'll see the most benefit. When you finally have an adequate brake back there you may find that your riding style changes a bit in order to get the most out of the fat tire. Particularly in my case with the greater traction of the ATV tire you tend to ride the bike with the back brake. Get out of shape, tap the back brake and the bike will stand up, stretch out straight and go on with life. You can use the rear like a sea anchor on really nasty downhills, too. If you can control your speed you'll never lose traction under engine braking like skinny tire will so all you gotta concern yourself with is steering. It's like a vacation once you learn it.
The 130 was standard on much faster and heavier Yamahas , so maybe it will be enough for your conversion. It's bound to be a major improvement, but if you trust my judgement
and can can afford the luxury of waiting til I'm finished with this I probably have enough experience to guesstimate how much or if you should just bypass it and go bigger.
I didn't go disc because we'd need something on the order of a 200mm disc to equal a 130 drum and there is all this parts interchangability between models. Less weight, no caliper mounts to fabricate, master cylinder and linkage to work out, etc. Mostly eyeball engineering.