Project X 5.0 in short irons 7 or 8-pw, then use project X 5.5 soft stepped two times in the longer clubs.

 Flex will be a tad softer in the longer irons and kick point moves up to simulate the flighted design. Some folks like the project X PXi , others including me hate the feel. 

If it's been a few years, it'd be best if you scheduled a fitting with your Titleist rep. Although similar, PX switched hands not too long ago, and they're not the same PX shafts.