Are you using more than 150GB worth of data on the 500GB drive?
If you format it at 250GB instead of 500GB you'll force the data on the fastest half of the drive and get better access times as well.
If you really want to push it you can format it even smaller and get better performance but if it's your only drive you may need more capacity.
The screenshots you posted look like it's running on a SATA II connection. Oh I see your burst rate is lower. Hmm maybe the drive is in SATA 1 compatibility mode? Looks like the HD503HI doesn't use jumpers for that so it'd be a BIOS setting if that is the case.
What motherboard are you using?
I don't really need all the storage on my drive, but I'm not really interested in short stroking it. I've heard mixed reviews. But I may experiment if I ever solve this issue. My board is a Foxconn RC410M01X1-6LRS2. Correct, no jumpers.