Rollover matters! You probably won’t see XC racers riding mullets anytime in the near future. Full 29 is just faster and a 29er XC bike is the fastest, which is what you were riding before. A mullet trailbike is not the fastest. Mullets are best for people with shorter legs and for tight twisty turns, not for going fast. However, before you trade in your Stumpy, try putting on some lighter faster-rolling XC-ish tires. I ride the Stumpy EVO which came with the same tires you are riding and I found them rather draggy and I’ve never liked 2.3 wide tires. I use the 2.6 Maxxis Rekon front and 2.6 Ikon rear or 2.4 Rekon rear. If you want to stick with Specialized, get a set of 2.35 Ground Controls. Maybe a set of Schwalbe Wicked Wills would do. Tires made a huge difference for me. I easily ride 10-15% faster than with the stock tires.
Also, spend some time making sure you’ve got the rear shock set correctly. Multiple reviewers complained the rear suspension on the Stumpy is difficult to set up. Too firm and the bike didn’t descend well. Too soft and the bike didn’t climb well. The Stumpy has a very narrow sweet spot. Some felt that they were never able to get it quite right.
If you do sell your bike, you sound like the ideal candidate for a Transition Spur, Ibis Ripley, or Rocky Mountain Element. Something a little more sporty. You could also convert your Stumpy back to full 29.