F-16 RCS is around 1/10th that of a F-15, which means detection range is reduced to 56% via-a-vis F-15. This lesser detection range is made up for by the much larger F-15 radar and/or AWACs support.
J-31 is VLO and is likely to come in at around 100 times less RCS than PAK-FA, which means that detection range will only be 31% as that of PAK-FA. Larger radar for PAK_FA and/or AWACs support is not going to make up the difference here.
The difference between PAK-FA and J-31 is pretty much like that between F-15 and F-16.
PAK-FA radar with GaN technology (5x times more power than normal AESA T/R modules), large aperture
and beamsteering agility will outstrip even F-22 radar. It easily offsets J-31's shaping. And you seem to forget
PAK-FA has L-band AESA radars on the wings which are way more effective at detecting stealthy targets
at long range than any X-band radar - although they have lesser resolution. Also there are side-facing radars.
Add to that PAK-FA's EODAS-like IRST apertures, interferometry-based passive tracking capability,
give it 100% passive operation capability when needed. Meaning, it can detect, track & possibly kill
you without even using radar. Stealth isn't just about RCS, it's about IR stealth and electromagnetic
stealth as well.
PAK-FA can carry 8 missiles in internal bays, J-31 carries only 50% that much, 4 missiles.
In terms of performance, J-31 is a dwarf compared to PAK-FA.
PAK-FA will get the Saturn Product-30 turbofans which are extremely powerful at 176 kN each, have
a very high T/W ratio and are Variable Cycle Engines (VCE). They have potential to become the most
advanced jet engines on any fighter aircraft in the forseeable future.
PAK-FA will even have F22-like shaped nozzles to reduce IR signature.
J-31 having 100 times lesser RCS than PAK-FA is not realistically possible. Shaping is important, but
only part of the game. Have you seen YF-23 Black Widow? It was actually more stealthy than
YF-22, despite it's top portion, air ducts, and engine fan blades being visible.
That tells you a lot about PAK-FA logic.
Just one sentence:
The Russians are building PAK-FA to take out F-22 and F-35. Shenyang J-31 is
not a match.