Mainly work longer, for sure, but if caps and resistors are replaced I can only assume SQ will improve too? I don't know, not too knowledgable at these things..
But the r2r2k black's SQ is something that I always saw as the summit (altho looking at daps like rs8, I've a feeling it's been surpassed, but that's something I'll try in the far future), so the only real issue I had was battery (even then, it wasn't too bad, it definitely was enough for a day if charged daily.)
I tried replacing different brands of resistors and capacitors and they all sounded different:
Resistors:
Vishay Dale RN55 Metalfilm - Pure clinical sound, somewhat synthetic, sharp and narrow.
Kiwame Carbon Film - magnetized caps, very noisy and distant sound, warm and colored.
Takman Carbon Films - noisy at first, but then clean, neutral and a little harsh.
Various old resistors from the USSR - carbon, metal, composite, all showed different bad results.
Shinkoh Tantalum - Musical and wide sound, closer, detailed with soft treble, a little warm, but in fact it is transparent and shows the warmth and musicality of R2R, the best on the market, but the remaining ones are sold out, the required denominations are no longer available, I had to solder in pairs.
Stock resistor - magnetized, sounds dirty and sharp on treble, and some instruments sound a little different.
After Shinkoh, all other resistors sound uninteresting. (Did not try Amtrans AMRG Carbon Film, original Riken Ohm Carbon and Audio Note Tantalum)
Capacitors:
Silver Mica Capacitor - Pure clinical sound, smooth but sharp on the treble, more high-frequency sound, slightly different tonality. The hoarseness of vocals on rock songs is lost.
Stock Ceramic Capacitor - magnetized, did not compare after removal.
Ceramic old Capacitors from the USSR - tried two types, one sounded good, the other had a narrower stage.
Glass-Ceramic old Capacitors from the USSR - The most open, transparent, wide sound, the best of all the capacitors I tested, but also the largest.
I would like to compare the mod with the stock and record on the line output, but I don't have quality equipment.