I ran all over the country in my last bus that was over 13'.
I had very few problems with height. The few issues that I did have were a bit of a hassle.
Like having to back up nearly a full block downtown Denver at 5:30pm on Friday because of a 12'6" bridge clearance.
People were so supportive. Waving, honking and telling me that I was #1 (sign language).
If you are 12' or less I would not expect any trouble. If you approach 13'6" you should not have many issues but you have to be much more cautious in your route planning.
I do have to ask, why a 27" raise?
I am raising my current bus 10" and that gives plenty of head room and a perception of space. I cannot imagine another 17".
I am not saying that a 27" raise is bad or wrong. I simply don't understand the need. I'm curious what the benefits are as opposed to an 8"-12" raise.