I have the same problem with 25/9 -- I am at ease with 25/8, no tension whatsoever, and when I get to 25/9, which everybody thinks is an "easy" etude, after only a few bars I get this pretty advanced tension in right wrist and forearm. The result is lousy sound control. The only thing I can do after 15min is to stop practicing 25/9 and resort to 10/11 or 25/1 for relaxation.
Please help, as I would love to be able to make this little gem sound as it should!