If your dryfire practice is effective then you don't need a MantisX.
If you feel like you have reached a plateau/aren't progressing, and your slowfire groups with centerfire handguns are still unacceptable, then the MantisX might be a good way to reach the next level.
As a general reference, if your trigger technique is good, you should be able to consistently empty a centerfire handgun into fist-sized groups or better at 15 yards shooting slowfire.
My least accurate 9mm handgun, shooting a mix of different ammo all loaded into one magazine will still put all its shots into 4.5" at 15 yards. Anything significantly larger than that and trigger technique/anticipation is the culprit.