What if the enemy fighter launches their BVR first and j10 has to perform high G evasive maneuvers with dual rails which will put immense burden on the pylon which is carrying dual rails.
We cannot what if these kind of things because in this case the enemy fighter cannot launch their BVR first. The PL-15 has better range and the J-10C has better radar than most fighter even 4.5 generation fighter. Su-35 with R-77 and R-37 doesn't stand a chance against J-10C in BVR. Typhoon doesn't even have AESA or even just a basic multi antenna radar. Rafale is much better and has Meteor so in 4.5 generation this is only one that can shoot at J-10C before J-10C can do anything.
Anyway what is the science to prove the dual rails deplete the performance and really by how much? I personally don't think the performance degrade that much. Also why does the J-10 need to immediately do evasive maneuver if the enemy already has upper hand? So in this case the one who shoot first always will win because the other will always immediately defend using evasive maneuver?
Whenever BVR fight starts, the pilots do not know they are already fired on with BVR missiles until missile seeker is active much closer. They usually understand the enemy's rough range and assume missile is probably on the way and most pilots will have to stay until they give their own shot.
I feel you don't understand how BVR fight works. It is difficult to understand but can say that range and radar are very important along with missile numbers but there is also fighter range, fuel, engine power, acceleration, climb, turning, RCS, electronic jamming etc.
To summarize more missiles is good and the J-10 can most likely only carry 4 BVR missile with 2 short range missiles and three fuel tanks. This is not good but also actually not bad load for single engine fighter. It is not F-16 level unless last pylon can actually carry BVR as well.
Consider the Rafale's and Typhoon's manufacturer own advertisement for air superiority is three tanks and 4 or 6 BVR missiles with 2 short range. This is at most two BVR more than J-10 with similar range on all three fighters. Both these fighters are larger and heavier class also much more expensive and complex with two engine designs. F-16 tip pylon should not carry AIM-120 due to damage to wings so F-16 with three tanks also can only carry at most 6 BVR with dual rail and 2 short range. This is better than the others but it is also carrying dual or triple rail. So it should also drop everything when J-16 fire PL-xx?
If things work this way, the AIM-54 or R-37 and PL-xx style long range missiles is all you need. Su-27 to 35 and J-11 to J-16 cannot be beaten and all airforce should just design only heavy weight. Missiles and radar are not perfect and no one really knows each other's electronic warfare technology levels well. J-10 designers really strangely decided not to give inner pylon missiles capability because for sure they know it is useless and will need to carry fuel tank anyway.