I know almost nothing about the 17 Mach 2 round. So let me get a few of those questions out of the way.
1.) Looks like the ammo used in the FN P90. So is it apt for home defense, unlike .22LR?
2.) Is it an expensive round? Less than 9mm but more than .22LR?
3.) Suppressable, and if so as quiet as .22LR?
Now for the 1911 conversion.
1.) Worth the buy?
2.) Better than a .22LR conversion?
I'm curious about this caliber myself.
It is more expensive, and I think a little bit better of a caliber than just .22
The 17HMR and the 17 Mach 2 are both rim fire rounds that use a bullet lighter than what a 22LR would shoot. To simplify it (maybe too much) the Mach 2 is a 22LR casing necked down to a 17 cal bullet. Both of the 17's are supposed to give improved velocity and performance over the comparable 22 rimfire cartridges.
To your questions:
2) I think you're close on cost. Last time I looked, 50 rounds of GOOD 17HMR was about $10.
3) Don' know, but don't think so.
On the conversion unit, it depends on what you want to do with the gun. If I were to buy a conversion I'd go 22LR simply to get more trigger time with less cost. That would be easier for me with the 22.