Well, let's assume....
According to the "Getting to Grips with Aircraft Performance" (a really nice publication from Airbus), TOGA takeoffs are slightly more fuel-efficient than FLEX takeoffs. As far as long-term effects of engine wear, yes, it's true, a tired engine burns more fuel...and the engine gets tired quicker when doing all TOGA takeoffs. So, I guess an argument could be made that, in the long term, TOGA takeoffs really are not much more fuel efficient compared to FLEX takeoffs. But, a discussion of this really starts to get banal.
Personally, I pretty much take what Airbus publishes as gospel truth.
Now, regarding the question concerning improved climbs.... Let's say, at a given weight, you have enough runway to do a FLEX takeoff with the maximum allowable FLEX temperature. And, let's say that, not only do you have enough runway to takeoff with maximum FLEX (under the conditions of weight, temperature, runway elevation, etc., etc.)...but, you still have thousands of feet of runway extra...on top of that.
So, If you have a zillion meters of runway to play with, but need only half a zillion to takeoff with max FLEX....and operations tells you that they want to load a couple of tons extra of freight....no problem...
So, you still may not be runway limited...because you have a zillion meters of runway at your disposal (accelerate-stop, or accelerate go to the end of the first segment....and, let's assume for this discussion that you're not tire speed limited)... But, now, you're second-segment climb limited with the flap setting you are planning to use that is related to your desired FLEX temperature and the other conditions as stated above.
OK...having said that....you could reduce your FLEX temperature....despite the fact that you have more than enough runway at the maximum FLEX temperature...you reduce the FLEX temperature to get a better second-segment climb at your new, heavier weight. (By the way, freight is a very profitable part of flying.)
Or, you could use up even more of the runway (which you have plenty to spare)....and get a 'jump-start' on the climb...to get an increased second-segment climb gradient...since, from your reading of the Airbus "Getting to Grips with Aircraft Performance" publication, you'll note that increasing the V2 speed will, to an extent, increase the angle-of-climb....again, "to an extent"...since, increasing the speed to 980 kts will give you less of a second-segment climb gradient that what you'll have with the improved climb situation.
So, if you have a lot more runway than you really, but are second-segment climb limited, and you still want to get as much FLEX as possible to save the engines...this is what you do.
All this assumes that you have RTOW charts for all of this...making it legal, safe, and OK to do without soiling your underpants.....
Fly safe,
PantLoad