Expectations after re cap of power-amp

Depends on the quality of the caps used. Could be quite a bit better than new. Talk to the tech, ask about how he selects caps. There is a huge range of performance- and yes you do pay for better.

Replaced all the electrolytics in a tube amp with 15 (hot) years on it, and it made a very nice sonic improvement - - all upside, no down, and no dramatic change in the overall voicing of the amp.  I hope you will be as pleased with your recapping.

My caps were not expensive high temp, low ESR Panasonics, FWIW.

I replaced all the caps in several vintage amps with Elna Silmic and Nichicon FG caps and the improvement was significant. For the $50-75 I invested it was well worth it. What amp could you buy for $500 that would compete power wise with the Yamaha?

  Fact is, that if the capacitors are not replaced, there is a (small) chance of big trouble. I am not trying to incite worry here, it is just a fact that capacitors do age. Normally, it is just the overall performance that suffers, but just like other components can, and might, fail in a way that is a game changer. Specifically if a component fails and then shorts out, all bets are off as to how much damage is accomplished. If on the other hand, the component goes open (no circuit connection), chances are better that no further harm will be done. 
 I often compare the care of audio equipment to a classic car. If you don't replace spark plugs when you should, it is not a good overall result for the engine. There comes a time when much more should be done to preserve that performance and value of the car itself.
 

Assuming nothing else has aged, yes.

I've heard good things from some who have had these caps replaced, meaning they experienced tighter, better bass.  If nothing else you'll have another several decades of listening to look forward to.