Shader compilers are extremely aggressive about unrolling since early HW often didn't have flow control and the cost on more recent HW can vary. If you have a benchmark you are actively testing against and a range of relevant hardware, then try things to see what happens. Your dynamic loop is more amenable to developer intervention than a static loop - but leaving it to the compiler is still good advice unless you have a benchmark available. With a benchmark, exploration is worthwhile (and fun).
BTW, the biggest loss with a dynamic loop on a GPU is that individual "threads" in a wavefront/warp will finish at different times. The threads that stop later force all the ones that finish early to execute NOPs.
Nested loops should be carefully thought through: I implemented a block based entropy decoder that encoded runs of zeros (for JPEG like compression). The natural implementation was to decode the runs in a tight inner loop - which meant often only one thread was making progress; by flattening the loop and explicitly testing in each thread if it was currently decoding a run or not, I kept all threads active through the fixed length loop (the decoded blocks were all the same size). If the threads were like CPU threads, the change would have been terrible, but on the GPU I was running on, I got a 6 fold increase in performance (which was still terrible - there weren't enough blocks to keep the GPU busy - but it was a proof of concept).
