Removing the midvalve shim stack and replacing it with a check plate is back-dating your forks 10 years !!
The '92 honda CR line was the first to use a midvalve shim stack, it didnt work too well, but that is where it all started.
The midvalve shim stack is one more area for suspension tuning, when the midvalve is set up correctly (either for woods or MX) it makes for a much better handling motorcycle.
Some suspension companies choose to remove the midvalve rather than test and refine, its a lot easier just to use an old set up, than it is to take the time and effort for getting the midvalve working properly. There is alot of suspension performance potential in the midvalve.
The DRZ400 comes with a check plate instead of a midvalve shim stack, owners usually complain of soft mushy forks, when a proper midvalve shim stack is installed it makes a huge difference.
I sent my 10 year old forks to Jeremy, and in talking to him he mentioned that part of the update being done to my forks was in the installation of a midvalve. It was my impression that this was done so that the initial part of the stroke could be kept plush and responsive while the midvalve would still help to provide bottoming resistance. I havent had the forks apart yet to see just what he did, but I do know that they are much more compliant and I am happy with the results.
Something needs to be done with the suspension on my YZ. The rider is 105 lbs and is c-level. I changed the springs from 41 to 37 and 4.6 to 4.2. The supension is now plusher but it seems the rear seems to be swapping around more. Enough that a fourth gear get-off in some bumps resulted in six weeks off from riding with an arm cast!:(
I was in the same boat with my son months ago. He is on a CR 125 that comes stock set up for a 180 lb rider. Once you go more than 1 rate up or down on springs the valving gets off. The "Fix" I was refering to was making the "mid-valve" work and not blow out like the stock YZ forks will do.
The MX-Tech guy seems to have the mod down, give him a try.