Can you measure spin up time for your 15k's? Some controllers can't catch the first D2H FIS after POR and have to wait unconditionally for spin up for hotplug & resuming from suspend. Currently 8 sec wait is used but it seems insufficient for your drives. Failing to spinup in that 8 secs would probably result in timeout of the first reset attempt and retrial - which currently takes > 30 secs.
Spin-up time can be measured by first issuing STANDBY and then time how long IDLE IMMEDIATE takes, I think.
The 15K RPM is a SCSI drive, I haven't seen any SATA 15K rpm drives yet.
I have the same model in another box, I can measure spinup with
START_STOP unit there. I will do so later today, when I fire it up, if
you still want to know?