With the advances in Avionics technology I seem to recall that the GR7 to GR9 upgrade actually lightened the aircraft whilst increasing the capability. Which was handy when the requirement to carry Sniper, a Terma pod and a useful weapon load was essential. Weight growth over the lifetime of an airframe is not a given, but more thrust is always good.
More thrust is indeed always good provided it doesn't mean less reliability and substantial higher fuel consumption, the F135 is probably scoring positive on all these fronts.
However the F35 airframe-strrength is already pretty much stretched as far as it can go for now, more power will probably mean more structural integraty needed.
Also for a 60000-70000 lbs it is very doubtfull that substantial weightloss can be achieved in the avionics department like in your GR7-9 example, certainly not looking at the complexity of the F35 in this department.
As an example it might be best to look at its predecessors, the F16 for one was specifically conceived with the idea that weightgain would have to be minimal over its complete lifetime.
It had intentionally very little empty room left in the beginning ,,2cu ft in comparison the F15 had about 15 cu ft of empty space in the beginning.
As a general rule fighters gain about a pound per day due to added complexity and added capabilities, the F16 scored pretty favorably on that point , the weightgain was limited to about 0.5lbs per day over its lifetime but even than it still managed to grow from a block 10 15600 pound fighter to a 19200pound block 50 fighter.
There is absolutely no foundation to believe that the F35 will not gain substantial weight over its lifetime.
A more powerful engine will inevitably mean some weightgain even further degrading the performance of the F35, wingloading is pretty terible at the beginning of its life and will further degrade when it becomes inevitably heavier.
Maybe the NAVY's proposal of going for the C iso A for the USAF wasn't such a bad idea at all, the larger wings will accomodate more fuel negating the need for something like CFT's in an already very bulky and draggy F35A.
Substantial weightloss for the Air Force C version could still be made by using a lighter landing gear , lighter arrestor hook+reinforcements needed to cope with carrier ops and using a boom-refuel point iso a heavier and more complex hose refuel system, also they could redo the gun and go for the lighter 27mm mauser single barrel iso the heavier 25mm gatling the A has now.
Less ammo would be needed also.