Numbers close to reality would require knowing the following: T/R module count, module output power, module duty cycle, module transmit losses, module receive losses, receiver sensitivity and radar level of digitization to name the biggest ones. We only know the module count and that gives us some clues but real world detection/tracking ranges we can only guess.
We know that AN/APG-81 has 1,676 GaAs T/R modules and that it’s most likely the most advanced fighter radar ever. AN/APG-77(v)1 likely has somewhat longer detection/tracking ranges but the difference is not huge if we assume very similar components and layout. Radar range equations give about 15 percent difference in favour of AN/APG-77v(1). AN/APG-81 likely is superior in many other aspects (A/G, EW, communications) as it’s newer design from the same company.
Comparison between two AESA radars made by different companies in different countries is impossible without knowing a lot of details about design and construction. If PAK FA radar and AN/APG-81 radar had identical components and layout, AN/APG-81 would have slightly longer detection/tracking range. Larger size of PAK FA radar would not matter much and higher number of modules would make up for smaller size of AN/APG-81 antenna. However, they definitely do not have identical components and layout. US has such a huge lead in designing and manufacturing components for X-band AESA radars and similar integrated circuits that it’s likely that AN/APG-81 has significantly longer effective ranges if similar values are compared (similar radar modes, similar targets, similar conditions). How much longer is impossible to tell, but the difference can be substantial depending on exact details of each radar. You have to remember that AN/APG-81 is fourth generation X-band AESA for US and third generation operational one. PAK FA will have first generation AESA for Russia.
I think 150 km detection range for 1 m^2 target is way too short for AN/APG-81’s maximum detection range. Older AN/APG-79 for Super Hornet is said to have 2-3 times longer ranges compared to AN/APG-73 used in Block 1 Super Hornets and that should compute to roughly 200-300 km detection range against 1 m^2 target. I’d say it’s likely that AN/APG-81 has maximum detection range of well over 250 km against 1 m^2 target and I would not be surprised if it could reach 400 km in cued search. Likewise detection range for AN/APG-77 is said to be above 200 km for 1m2 RCS, though actualities we may never know.
Of course the range depends on how large part of the sky is scanned and with large search areas the range is lower as the radar can’t use a lot of time to scan any one part. Also tracking large number of simultaneous targets could also affect the search range. Detection range numbers differ greatly depending on many variables and can easily lead to apples to oranges type comparison.