but the answer to your question . . . .
Will the fan RPM increase (clutch engages) to match engine RPM if the engine RPM is already at around 2500? . . . . is
No!
The fan(s) only operate to 2000rpm area not matter what the engine/radiator temperature. I think you said that in the first paragraph.
But you are incorrect re the temperature at which the vfc engages: for the W140 and W129 cars, it's very high, as I said, about 110degC. One could ask, why bother since it's already a bit late!
Check my Menu #21 to see a test of some of the Sach's newer vfc fans. I was looking for a fan clutch to replace my W140's vfc but couldn't find one.
If you search here, you will find that Sachs
does make a newer version vfc that operates at 90C or so. Check out this thread which is very informative and has pics. Here's the link:
http://www.peachparts.com/shopforum/showthread.php?threadid=43792