My 2 cents:
I think a lot of the complaints about reset exist because of Glock shooters. There are a lot of people out there who have cut their pistol shooting teeth on the Glock trigger and they’ve come to define that trigger as the trigger and every other gun they pick up gets compared to it. Thus they pick up a gun and try the trigger and immediately get that baby-eating-strained-peas yuk face because Lois, this is NOT my batman glass…erm…Glock.
Because of that, I think complaints about reset are somewhat overblown. Few other pistols on the market have the sproingy, ultra-clicky reset of a Glock, but that doesn’t seem to get brought up much either because a lot of people have limited experience with other handguns or because they just don’t think about those pistols in comparison to a Glock like they do with the M&P. I also think that the reset complaints get to be a bit groupthinky at times…as in somebody mentions it and it gets parroted around and around and around until I’m hearing in the gunstore trying to buy an M&P and I’m hearing word for word comments I read in a thread on ARFCOM a couple of days before.
…now that’s not to say that people are wrong for having trigger preferences. If they have done most of their learning on a Glock trigger and want to keep that Glock feel because that’s what they are used to, it’s fine by me. I just don’t think it should be morphed into a requirement that every trigger on the planet must feel like a Glock trigger or the gun isn’t suitable for use.
Now all of that being said:
How important is the sensation or audible “click” of trigger reset to the individual shooter?
If the person learned to shoot on a Glock…and by “learned to shoot” I mean someone who has done most of their formal training on a Glock…then that sproingy, super-clicky reset is probably going to be very important to them.
If they have been shooting other guns, it probably won’t be as important to them, although they may still welcome it.
Personally speaking, I learned to reset the M&P trigger the same way I learned to reset the Glock trigger…with dryfire practice at home. With a stock M&P and the trademark anemic reset of a stock TRS I can still manipulate the trigger FAR faster than I can actually intelligently direct the bullets into a target. The M&P’s reset, even on a stock gun, isn’t an obstacle for me. As long as the reset is reasonably fast, I can find the reset point and run with it. I’ve got 6 M&P’s right now and 5 different trigger configurations on those M&P’s currently that range from bone stock to a full custom trigger job to Apex’d guns. Some have a more tactile reset than others. All save two have Massachussetts TRS in them because I like a little more resistance in the takeup and because it makes the trigger reset faster, making a short-stroke very unlikely even if I’m moving my trigger finger at barely-on-the-berm speed.
Having owned and shot M&P’s for years, I see no compelling need for such an item.
But:
is there really a market for such a part?
Probably. While there is no compelling need for such a device, many M&P owners may prefer the feel of a sproingy, ultra-clicky reset. Assuming the device is priced reasonably and that it’s relatively easy to install, you might find a market among them. You may also find a market among those who pine for the feel of a Glock trigger when they shoot an M&P. I’ve talked with several individuals who love everything about the M&P better than their Glock except the trigger reset. If they could spend 30-50 bucks and 15 minutes fixing that, they’d probably be very happy individuals. If it costs much more than that or has installation that would test the patience of Job, it’s unlikely you’ll sell many.
If you stick to more or less the model you’ve established with your other parts, you’ll probably sell a fair number of them…at least until S&W figures out what you’re doing and rips off the idea. 
