A clarification to start: The Dynalite Gold which I mentioned (in this thread) is a classic True Temper shaft that weighs the same as the Dynamic Gold. Main difference: Dynalite was high launch, Dynamic was low launch . It appears TT has lightened the Dynalite shaft, and now has three vesions of it. The FCM comparison table I used is a couple of years old, and plotted the old Dynalite Gold rather than the new versions.
TT considers the PX 5.5 a firm shaft, which is between Regular and Stiff. The 5.5 has an FCM rating of 6.0, whereas a DG S300 has FCM = 5.8. Thus, the PX 5.5 is a touch stiffer than the Dynamic Gold S300. For the DG R300, FCM = 4.8.
I play the PX 5.0 Rifle Flighted in my X20 Tours. One reason I dared to get them was the 5.0 flex; Callaway Preowned has a bunch of X20 Tours in PX 6.0, just too stiff for most golfers. The PX 5.0 has FCM = 5.5 and weighs an average of 110 grams once installed, but feels a little stiff on days I'm not swinging well. With shafts below 95 grams, however, I spray the ball all over the place.
It comes down to clubhead speed - I have driver speed of about 85 mph, R-flex range - and how you load your hands (ask a pro about loading).
Being you've tried Mizuno, what did their Shaft Optimizer recommend for you?
Since you use 6i-PW in the X22s, you might consider reshafting. From your comments, you seem to be on the line between Regular and Stiff for the Dynamic Golds. What you might do is have a clubfitter reshaft your clubs in DG S300, and softstep them, or in DG R300 and hardstep them. Stepping gives you an in-between flex - the fitter can explain the details.
Reshaft estimate would be ($18 for DG shaft, $12 for reshaft labor, $7 for new grip), about $37 x 5 clubs = $185. Any chance could get a trade of some kind on X22s and just get fitted for new irons? (The X22 irons came out in 2009, and were not the easiest to hit).
As for shaft length and lie angle, that varies from club model to model. Not easy to transfer specs between club manufacturers.