1. Your current velocity isn't really that much off the mark for other 124s out there, and it's good to keep in mind that there is some variance from one rifle to the next of the same make and model number. I'm trying to say that you could spend a lot of time chasing that last 30 fps and never get it. So, what's it worth to you? Is the lack of that 30 fps. keeping you from doing something you need to do? (Edit: Oops. It looks like it could actually be more like 60+ fps. lower than some others, so I can see your point. I misread it the first time--sorry).
2. What Mark said about some remaining old piston seal debris is often right on the money when it comes to the 124. Of all the air guns on the market, the 124 is notorious for having a piston seal that disintegrated and cluttered-up the compression chamber when it went bad. Seeing it, then removing it can be a daunting task. When I tuned a 124, almost all of them had a ring of old seal debris at the end of the compression chamber closest to the breech. The beige color of the factory seal material tended to blend in with the color of the chamber. Even using a Mini-Mag flashlight (which will nicely fit inside the tube), you might not see the debris that needs removed to get complete piston travel and thus full velocity. Sometimes I had to use one of those little fiber optic attachments on the Mini-Mag light (see link below) to focus all around the perimeter of the chamber to see what was there (or not there). Then it took using the right tools to get into the edge of the chamber and remove the debris, even when I saw it. As long as you have the rifle taken apart again anyway, I suggest you look for any such debris and remove it if it's present.
3. Your question about removing that little hump on the face of the seal has some merit as it pertains to velocity, but probably not by a lot. You can sand it to a flat surface and probably pick up a few fps. Just keep in mind that the original design of the seal (with the hump) might have added a little cushion to the shot cycle as the piston landed on the chamber wall at the end of its travel. How much? I don't know. I can tell you that the originally-designed piston seal for the R1 had the same hump on it, and when it was removed the velocity of the rifle increased but the shot cycle didn't become anymore harsh to a noticeable degree. Now, as far as to going further and cupping or concave-ing the face of the seal as you mentioned, you may be in some uncharted waters on the 124 with that. The piston seal on the 124 has a slotted-parachute design versus a non-slotted-parachute design and that could make a difference. I don't want to type all of the negative possibilities, but although it works well on some other air rifles, there could be some negative effects and you might actually lose some velocity and need to order another seal and start over.
I hope that helps. Best wishes on your project.
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.