I wouldn’t say it’s painful compared to maya, there are ups and downs, but they are more or less on par when you put it all together.
I can’t speak for him, but I think Mocaw had more C4D than Maya in mind when he mentioned there are other more modern toolsets for hair.
C4D’s hair module is indeed more modern (I only had it demoed to me and played with it for minutes though), whether that’s enough for you to shift the balance though I don’t know, there are other things that would put me off it when it comes to feathering a creature, or for character work in general, but my opinion is only half formed.
Btw, it’s also worth noting that while the hair tools in xsi (grooming and gen particularly) might be getting stale, strands and strands interpolation in ICE are pretty bloody powerful, and there isn’t an equivalent offer in terms of power and proceduralism in any other package on the market except Houdini, which would set you back a fair deal more than XSI though.
Rendering wise, it depends what you compare it to. Scalability wise, especially on a 64bit platform, MRay does ok, definitely better than LW, 3delight though is even better, but that’s additional cash, a fair bit of it if you need to support many cores. The first license of 3del is free even for commercial use, so it might be worth a go, but I don’t know (don’t think it does) it includes a free license of 3delight for xsi too, so you’d have to resort to free but more technical/tinkery solutions like Affogato.