If we consider the average RCS of 0.3-0.4m2 for PAK-FA and 3-5 m2 for Su-35 with weapons then it gives an advantage of early detection for PAK-FA and first launch of BVR missile , ofcourse the ESM of Su-35 will warn of such BVR launch and it would take evasive measure.
It lets the pilot of the PAK FA decide whether to fight or to leave.
If he is fully fuelled and armed he can manouver to the side or rear of the the Su-35 staying out of sensor range and then creep up behind him and launch a couple of missiles at his rear.
Alternatively he could hold back and fire an R-37M and then wait 30 seconds and then fire another, and then wait another 30 seconds and fire a third missile. While the Flanker is manouvering and jamming and doing all sorts of things to deal with the first missile the second missile will be lining him up and if they both miss then the last missile will have a good chance for a kill too... and while this is happening the PAK FA could close in to launch an IR missile or leave the area.
Remember when trading missiles even if the Su-35 can detect the PAK FA at 100km, if they start trading missiles the missile seekers will have reduced range performance against a low RCS target so a BVR missile might blow past a PAK FA without detecting it.
Besides, a decent motor can overcome lack of lift from control fins to a degree.
Lack of aerodynamic surfaces means it will likely rely on hypersonic body lift... as it slows down there will be a dramatic loss in manouver performance because of a lack of control surfaces and of course because when there is no thrust the thrust vectoring is useless too.
No big deal however as IR WVR missiles are generally not used at ultra long range... 20km range targets are normally engaged with AMRAAM or similar.
BTW what is the possibility of a R-37M and modernised Mig-31 intercepting a Mach 3 Brahmos/Yakhont ? The R-37/Mig-31 is described as capable of intercepting target with a maximum speed of Mach 6.
Depends on what part of flight it intercepts it and from which direction. A manouvering low altitude missile would not be easy, but a cruising high altitude missile should be well within design parameters.
Of course scramjet Brahmos should exceed the mach 6 limit, but of course by then we will likely have seen the replacement for the R-37.
Honestly I doubt it would be outside the capability of the system, but it wouldn't be as "easy" as firing at an E-3C or U-2, which are significantly slower.
The fact that the Mig-31/R-27 combo are described as being capable of engaging Mach 6 targets suggests a mach 3 missile should be well within engagement parameters.
Of course having said that a Mig-25 should be a relatively easy target for AAMs as its manouver capability is limited to 5gs, yet they can be difficult targets with a skillful pilot.