The Powers of the Istari

[Raynor]

I interpret "far higher order" meaning that Sauron was a far greater Maia than Gandalf or Saruman.

I believe that a "far higher order" would reffer to the difference between an embodied istar and Sauron; an istar would assume many weaknesses specific to a Man's body (cf. The Istari, UT), and the "needs to learn much anew by slow experience".

Not only are Gandalf and Sauron "of the same order" here, but Gandalf "might be expected to master Sauron."

I believe it is relevant to mention the two reasons given in the letter for this:

It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors.

It would be interesting to explore what "superior strength" implies. Galadriel says that a ring would give power according to the measure of its (or at least after a while, if the wielder is not powerful enough); we would also need to speculate, if possible at all, how much Sauron's corruptions diminished his power.