@rohitvats, answering your questions one by one...
While AWACS and long range AAMs are a buzz-word these days - how will the fighters fare at this altitude? IAF fighters taking off from virtual sea-level and PLAAF taking from 10K+ feet airbases.
Leh and other high-alt bases apart, the IAF has a definite advantage across the board at all its bases. Taking off from almost sea level both in the Gangetic and Brahmaputra basin, IAF fighters can do full-load take offs with very efficient runway length utilization. Even hot days are manageable because of the high density of the air.
The PLAAF on the other hand has a fight on its hands at bases above 8,000 ft. Aircraft will require a lot of runway to pull off loaded take offs, often will have to sacrifice weapon or fuel loads to make best use of available runways. Things will be especially bad between April to August with high day temperatures affecting the density of the air.
- in terms of air-combat performance in the 20k-30k feet (ASL) sub-set, how do Indian and PLAAF aircraft fare?
Answering this from a pure air-to-air engagement perspective, 25,000 ft to 45,000 ft is kind of the sweet spot for long range air engagements for gen 4++ aircraft. Turbofan engines in today's A2A fighters are optimised for this altitude regime, this applies to both the IAF and the PLAAF. Launching missiles in this region offers good kinematic performance. (Gen 5 -- Raptor, PAK-FA etc are optimised for 55,000 ft plus offering its own advantages but that's beyond the scope of this question)
The biggest catch I see is the high floor of the plateau pushing engagements to the higher end of that band to allow for more space to pull up from evasive maneuvers. Pulling up at 15,000 ft is not the same as 5,000 ft. Also pilots would need to watch out for low-level flak and radar directed guns that are placed on the high floor. These guns are typically good till 6,000 ft, so if they are at 10,000 ft, you'd better not venture below 16,000 ft. (If you recall in one of Vivek Ahuja's scenarios a Su-30MKI gets hit by flak while pulling up over Manasarovar lake.) Also from what we saw in Kargil MANPADS work really well in the cold rarefied air, and they can engage upto 9,000 ft. These things will have to be factored into while planning air to air ops.
Now, the valley floor is at 14K-15K feet - I am assuming that any ground attack on targets in areas like this will have to commence from 25K-30K feet bracket.
Definitely if you want to use LGBs you would look at dropping the bombs from 25,000 ft+. Here again bomb kits need to be calibrated for the higher altitude release and corresponding adjustments might also be need to made on aircraft avionics. But we have knowledge of this from Kargil. Operating at a higher altitude with a full bomb and fuel load will not matter for the aircraft if its engines are optimised for this regime. This is the reason why the Jags are getting new engines.