The 9mm Hi Power comes with a factory standard recoil spring of 17-lbs. I prefer to increase this weight to 18.5-lbs. Heavier springs are available, but this one works fine for me with both standard pressure loads as well as the hotter stuff. I do it for two reasons: One is two reduce battering between the slide and the frame while the other is to possibly reduce potential rounding of the barrel's locking lugs.
If you shoot primarily standard pressure ammunition, it's probably not an issue, particularly with the Mk III Hi Powers and the models based on them. If you shoot warmer loads frequently or if you use warmish handloaded ammunition, I think the slightly stronger recoil springs serve a purpose in the Hi Power's long-term useful life span. The heavier spring in combination with the 32-lb. mainspring (hammer spring) will slow down the unlocking process when the pistol is fired. In using these springs over several years in several Hi Powers, I have never seen a problem caused by their use. The only standard pressure load that I've had a solitary failure to eject with has been the 65-gr Aguila "IQ" round. Everything else has functioned fine. Should this be your load of choice, by all means go with the standard power recoil spring.
Some have opined that the heavier spring will wear the gun as much as hot ammo when the slide moves forward and that we can either have possible damage from hot loads with the standard spring or increased wear on the pistol from the slide slamming forward with more velocity from the slightly heavier spring. This seems reasonable, but I don't think so. Here's why. The forty-caliber Hi Power uses a 20-lb. recoil spring for the more powerful .40 round. While the forty Hi Power's slide is slightly heavier, this is twice the increase in recoil spring strength provided by the 18.5-lb spring I recommend with the Hi Power. The forty slide is not even close to being twice as heavy as the 9mm slide. The sear, sear springs, and hammers on both are identical. Evidently FN does not think that this extra forward momentum of the slide closing causes any problems. Remember that the slide's forward motion is slightly retarded as it strips a round from the spring-loaded magazine during this step in the feeding cycle. FN also included another locking lug on the forty barrel to increase the total surface area engagement between the locking lugs and the slide.
Another example would be the 1911 pistol. The 10mm Delta Elite has a considerably stronger recoil spring than does the .45 ACP, yet the internals are the same in both calibers. One can see the same thing in the CZ-75 pistols chambered in 9mm vs. .40 S&W.
Where I do think that the increased strength recoil spring might eventually cause problems is if the slide is allowed to repeatedly slam shut on an empty chamber.
The only other possible downside I can see to the slightly increased strength recoil spring in the 9mm Hi Power would be if the pistol were being shot with a significantly weaker grip or hold. Such is certainly possible in a defensive situation, as the shooter might have been shot in the gun arm or just not have a really firm hold on the gun. Semiautomatics must have a certain amount of resistance provided by the grip for them to function. The frame must be held strongly enough that slide's rearward momentum compresses the recoil spring and doesn't move the frame rearward instead. I have noted no incidents where this has occurred, but it seems possible. If this is a worry, just use the 18.5-lb spring for the range and practice and use the 17-lb. for carry or when the pistol's available for protective use. In my own defensive 9mm Hi Powers, I use the 18.5-lb. recoil spring exclusively.
The few Hi Powers I've seen with damaged locking lugs also had worn out and overly weak recoil springs. I suspect that despite the guns having been shot lots for years, their recoil springs had never been replaced! Some owners admitted that this was the case while others said that they "couldn't remember"...