Ok, I have to report back with my success now that I have a minute, I haven't been on here for a while. I confirmed the replacement flasher relays work perfectly in my 2012.
I got this one from Amazon/iJDMTOY
Amazon.com: 8-Pin Flashing Speed Adjustable Electronic LED Flasher Assy Relay Fix For Lexus Scion Toyota LED Turn Signal Light Bulbs: Automotive
It's the adjustable one, but they have the regular one too. I figured I'd get that one so that I don't get stuck with a blink rate I don't like or something. It works as advertised. Once I got the hang of removing the plastic panel it was pretty easy.
So I went to DiodeDynamics and swapped the backup light LEDs I got that I didn't like for another set of blinker LEDs for the rear blinkers - I was pretty happy with the front ones and I decided not to go with switchbacks for various reasons.
When I got the chance, I installed the new rear blinker LEDs. They hyperblinked of course. So then I swapped out the flasher I mentioned above - took a little finger wrenching in a tight spot. Problem solved! Adjustable blink, lol. At that point I pulled out the resistors (and my nice mounting and electrical tape job) I had put in for the front blinkers. Thankfully they were little splicer things that didn't actually cut the wire - big help. Everything worked great, and no hot resistors. So I have 4 11watt LED blinker bulbs running on that flasher. Now the replacement flasher says max 150 watt so you might have to watch it if you go really bright. DiodeDynamics for instance has an 80 watt option for their brightest blinker LEDs so you'd be going over the limit with just two of those.
I kept up with the DiodeDynamics guys and said I'd let them know how it goes, so I told them about the success and they said thanks and they'd pass it on. Hopefully they update their product for the 2011+ xB now to allow for the flasher option.
I really like the instant-on-off of these, probably like any LEDs and like the side mirror markers. Also, you don't see the yellow "egg yolk" there anymore so especially for the front of the car it looks cleaner, more silver and clear. I really didn't think I'd care about that at first but I do notice it now. There is a slight delay when you flip the blinker - the blinkers light immediately, but the first blink is slightly longer, no big deal at all. The LEDs I got are subjectively maybe 50% brighter than stock. They stick out further into the reflector housing so they probably don't reflect as nicely as the little bulbs did, but the brightness more than makes up for it. Plus there is a projector over the LEDs on the end of the thing which definitely helps.
Next up, brake light LEDs probably, hehe. Instant on/off will be good for those.