Overblows, Overdraws and Tight Gaps
"Gapping almost any harp will allow you to play overblows!"
True. But let's be specific about what we mean.
Almost any harp can play an overblow that you "wind up"; it's easy to play the overblow when rolling off the draw note. This isn't as big an achievement as some seem to think it is. Those kinds of overblows are very limited in their application because more often than not, you want to be able to play the overblow on its own as a single note.
A true test of a harp's "overbend-ability" is whether you can play the overblow from a dead stop. Gapping a stock harp tightly will not reliably allow you to play overbends from a dead stop.
The other thing about "just gapping" a harp for overblows is that gapping is a very crude way to adjust reeds. It only takes into consideration the height of the tip of the reed but it's the height along whole length of the reed that is crucial. In essence you need to consider the shape of the reed.
If you don't, sure you can slam down the gaps and crank out an overbend, but very likely, that's made the hole miserable to play. The instrument will only play with light breath, low volume and weak tone.
On the other hand, an Overbend harmonica with proper reed work will offer you full range from quiet to loud, rich tone and the regular notes, bends and overbends, all play with the same effort.
Reed work can increase the available range while making the harp much more responsive. It should be more fun to play an overbend harp, not less.