The Powerblock Sport 5.0 (or 5.5) Stage II Kit is a little bit of a unicorn. Look for it on the Powerblock site and you won’t find it. Call Powerblock and — so I am told — they will tell you the only upgrade available for these blocks is the heavier core for $150.
Well some time earlier this year, in the February/March time frame, someone mentioned these to me as a comment on YouTube. I had not heard of this kit before, so I checked it out, and lo and behold, it was on clearance sale direct from Powerblock for $79! I immediately called up my local dealer (the same one I had used to buy the blocks) to see if they could order them for me (and save me the cost of shipping). Fast forward about three weeks, and now my Sport 5.5s go up to about 80ish pounds. Awesome!