Is it improper to ask to speak with previous patients who have had revision rhinoplasty done by the surgeon I am considering for the same procedure?

Not at all!  And you absolutely should.  All surgeons should have a referral list  of prior patients to discuss particulars and trade stories.  I personally spoke with patients beforehand and as a matter of fact am on my own doctor's referral list.

If you are wondering if all of the patients are going to be happy or receive compensation... I have found in my experience that there are about 1 to 2 patients who wish they had done more research beforehand or had gone to another doctor entirely so be advised that although this is unfortunate I think it is helpful to the patient considering the procedure to gain a well-rounded opinion.