I've done the same thing to my Striple (except I have a Rizoma reservoir) - and now regret it. Either my m/c or my calipers need rebuilt and I have yet to locate a rebuild kit for the m/c. I've been chasing down a soft brake lever issue for 2 years, must have bled the darn thing at least 50 times and have come to the finality that one or the other, or both, needs rebuilt. At least the kit for the Nissin radial is easy to get.
I was a bit concerned before the install and part of the reason I held off for so long. On my D675R I experienced the initial lever pull to be mushy after the bike has sat over time (an hour, overnight, etc). After the initial lever pull, the brakes were solid. I did resolve that issue eventually, but I don't know if it was from bleeding the brakes or going back to the stock lever because they were done at the same time. Bled the brakes a couple times prior to that and didn't work so that leads me to believe it was the aftermarket lever. I didn't have that issue prior to the aftermarket lever either; but then again there was a forum member who had that issue with a stock lever.
Fast forward to having the M/C installed on my RX and the initial lever pull is solid after the bike has sat for some time. Took the bike out for a ride yesterday and all was good too. Seems to take less effort pulling on the brake lever, but won't really be able to test the difference until I get her to some twisties. Only time will tell whether I'll experience the same issue from before, but I had several trackdays on my D675R without issue so I'm optimistic that it will last.
Your post made me think a lot about my immediate M/C purchase plans... Now, next for me this winter was a Brembo 19 RCS, and I was ready to pull the plug on it, I mean within a day or two. Now I started wondering about availability of rebuild kit down the road- knowing myself, at the first signs of loss of stopping power, I would add rebuilding the M/C as well to the winter maintenance list, especially that you just said the kits for the stocker nissin are readily available...something to research before deciding whether to buy the brembo or not.
So far no issues for me with my D675R OE Brembo M/C on my ST3RX. As for swapping to Brembo, I did it because the M/C was at my disposal. I think the OEM Nissins on my RX are solid, but wanted to see if there was an improvement with the Brembo M/C.
The 19RCS is an excellent M/C as that's what I'm currently running on my D675R. That's the 3rd bike I've used it on (same M/C) and haven't rebuilt it yet. On top of that, I picked it up second hand in excellent condition. I did swap out the rubber boot for the push rod because it was torn, but that's about it. I do applaud your efforts in rebuilding every winter though.
For the track I wouldn't hesitate to pick up the 19RCS, but for the street, I'd say it isn't necessary. We do have our vices though; I'm probably going to upgrade pads and rotors in the future.