My pleasure! For an advanced course, Toto obtains fill valves from two or three suppliers, depending on which plant your toilet is built in. A lot of the US-built and Mexico-built Toto toilets will have a Korky fill valve inside. It is usually set at the Korky factory to the proper refill ratio, that is, the percentage of the water coming into the valve that goes down the overflow riser to refill the bowl. Toto's toilets vary on the refill ratio, usually between 28 and 40 percent. So those will have a green cap or a blue cap or some other cap. The valves are basically-identical inside, except that there's a little device inside that controls and sets the refill percentage and isn't user-changeable. The 528MaxPerformance brings the little device that sets the refill percentage to a spot outside the valve, so the user can set it with a little knob, between zero and 40 percent. So that's why we recommend it.
Here's the advanced-student part: If your current fill valve is shaped more-or-less like this:
http://www.lowes.com/pd_336988-868-528MPK_0__?Ntt=528mpk&UserSearch=528mpk&productId=3284948&rpp=32 then it's a Korky-made fill valve.
ALL the Korky 528s (and they have a few models, including the custom ones they make for Toto), have the moving parts, the seal, contained in a little cap that looks like this:
http://www.homedepot.com/p/Korky-Toilet-Fill-Valve-Replacement-Cap-R528CM/202666007?keyword=r528 They all use the same little interior cap.
If you have an issue with the valve, it can almost-certainly be solved by replacing that little cap for $4. It takes less than 5 minutes to do so, and you don't have to remove the valve from the toilet. You can also "service" the valve by cleaning it at that time, and there's a video on how to do so.
The other valves that Toto uses don't have this feature, so that's why it's nice to replace it with the 528MaxPerformance when it comes time to do so. Toto actually has a part that it sells, TSU99A.X, that is essentially a 528MP, and they call it Universal Replacement Fill Valve for Toto toilets. It's effectively the same thing, so you might as well just get the part at Lowe's for 2/3 the price (or less).
Hope this helps.
PS Oddly, Home Depot sells a number of Korky fill valves, and sells the little R528 cap, but does NOT sell the 528MaxPerformance. Lowe's sells the 528MP (or MPK they call it), but doesn't sell that little R528 cap. Go figure.