Ken, Here is what I think.
I agree with going back to the basics. As I have been reading this thread since your project started, I for 1 am not real sure where you are with methods, hardware, program,...

I suggest you do this: List all the parameters needed. such as how long does the sonar need to accquire a distance? How many things does the PIC need to do? Then you will be able to determine if your chosen PIC is up to the job.

I think it prolly is, and I think PBP is also up to it. Worst case is to sprinkle some ASM in if needed.