Re: LUN with high response times ? How to fix?
Using the "high then auto" policy only means that new slices will be obtained from the highest tier possible and then will be auto-tiered to the correct tier (as deemed appropriate by FAST on the array) at the first opportunity. If this LUN is not constantly getting new slices that may not help much. It probably wouldn't hurt either but it most likely wouldn't help much more than just having it set to "auto-auto".
Have you noticed any other signs of bad performance - forced flushing, queue full counts on the SP ports, high SP utilization, etc.?