"While more and more people started putting fancy ultra smooth ball bearing bushings into their gearboxes these days. When it comes to pure reliability and durability, nothing can beat the good old solid bushings. Ball bearing bushings, while do offer nearly friction-less movements, are by their nature much more fragile compared to the traditional solid bushings. Especially if you use ball bearing bushings in combination with high tension springs (M110+).
When ball bearing bushings start to fail, due to their design, the axle will slowly start shifting off center, causing the gears to start rubbing and bumping each other and the gearbox at odd angles. What's worse, the user generally don't notice this happening until their gearbox starts making audible noises. But by then it's generally deteriorated to the point, that not only do they need to replace the bushings, but also the gears, and possibly end up with a beat up gearbox as well.
While solid steel bushings, with no moving parts what-so-ever, simply don't fail at all, ever. Your gears would break long before these give up. And when the gears do break, they'll still be at perfectly correct positions to each other and the gearbox, and most likely all you need to do is to just clean out the broken teeth in the gearbox, and then replace the gears.
Yes, they do make ball bearing bushings stronger and stronger these days, but I personally have used this particular sets of steel bushing, and perhaps because of the oil channels built into them, the gears can free spin about as smooth as with any ball bearing bushing I've seen. So in my opinion there's really little reason to risk possible catastrophic failure that you'll only notice when it's too late, for just a possibility of a tiny bit of RPS increase.
But do take note that these will not fit into King Arms P90 ver.6 gearboxes.