I'm an 8.5 HC with a driver club speed of 103MPH.
I haven't hit them yet. Reading about how these shafts may be a bit flexible for my swing, but unsure.
What do you think?
First of all, welcome to TST!
The MP53s are really nice irons, I can tell you that much. I haven't hit the PXi myself but I know that they are a pretty tip stiff design, though they have reduced overall weight compared to the original PX. This means they'll be fairly low launching compared to other light shafts, but not quite as much as the PX of the same flex. I don't think they'd be unusable for someone who fits into the PX 5.5, they'd just play a little softer which can be a good thing in some cases. They seem to fit about the same swing speed range but the desired flight and weight characteristics are different.
The PXi is supposed to be pretty good on the longer clubs, as the extra launch and speed helps with the long irons. They can also be played in slightly longer lengths with the same swingweight, which can add a bit of distance. I think a driver SS of 103 tends around low 80s for iron swing speed, so assuming the profile of the PXi fits you a 5.5 flex sounds around where you want it. The 5.5 plays around a true stiff flex compared to shafts such as the Dynamic Gold. If you played S300 in the past then the 5.5 should fit, though the PXi is lighter.
You don't want to be picking too strong a flex in your irons. If you want an X stiff driver with a tweener swing speed, you can get away with it somewhat, in fact many companies make their wood shafts play a bit soft to flex because a lot of guys go up in flex that might not need to. In irons though, the accuracy from a fitted shaft is a big help. I would try the 5.5 and see if you like them, since I think you'd be a 6.0 at absolute most. Based on what I've seen, a 6.5 on the PX scale is a true X stiff flex, which probably would require an iron swing speed of the low 90s in most cases to work best. Shaft companies don't always recommend shafts by swing speed anymore, since it's only one aspect of fitting, but generally 5.5 should be on the radar for someone in the low 80s as a true stiff flex.
If you don't like those shafts, they're pretty easy to pull and replace with something else. I wouldn't expect to change flex by much, but if you do think they're too flexible, I'd try different weights and profile shafts before jumping up a flex. I also can tell you I love KBS shafts and they should be worth a look if you shop for something different.
Also, have you had success or a high comfort level with a different iron shaft model in the past? If that shaft profile was a lot different, the transition may not be as easy at first. If we know what you're used to it's easier to make comparisons.