It depends on who you talk to. My understanding is that Project X 5.5 is considered a bit stiffer than a regular flex and 6.0 is close to a typical stiff flex. The shaft is a different profile so you should take the old shaft with you to the demo and do a side by side comparison, unless you can go to a fitting and review all the shafts offered.

It depends on the shaft. 6.0 iron shaft is different than 6.0 wood shaft. For the HZRDUS lineup, 5.5 is R, 6.0 is S and 6.5 is X.

According to a recent True Temper Video Project X 6.0 is stiff in both woods and irons. Other wood flexes are Lite (4.0), A (5.0), R (5.5), X (6.5) and Tour X (6.5Tx). Other Steel Flexes are: R (5.0), R+ (5.5), X (6.5), and X+ (7.0).

So as you can see in the Project X shaft, 5.5 in graphite shafts is regular, but, is R+ in steel so this can become confusing. Further complicating is that the Motore Speeder stiff could be a stiffer or softer shaft than the Project X shaft. Shaft manufacturers all have different methods in determining what is regular, stiff etc. so you just can't assume relative stiffness between shafts based on the Flex letter they apply to a shaft.

As Barry states side by side comparison is the best way to proceed and/or a complete fitting.

Yes, 6.0 is akin to a traditional stiff. I have the HZRDUS smoke 6.0 in my TS3 driver and hybrids, they're fantastic shafts.