Actually, hold on, I wonder if seek would improve very noticeably? Is seek time mostly the long-distance movements of the head, or the locating of the file in-track after the head has traveled? If it is the latter, you might be able to boost seek quite a lot.
Though what you're really going for here is "access time", which is "seek time" (properly defined as just the lateral movement of the head) + "rotation time" (what you call "locating of the file", which is actually just sitting there waiting for your 7,2k rpm to come around once every 4ms or so - rotating disks rotate at a constant rate, you can just do the math). A Western Digital Caviar Blue (random benchmark, I only know this because it was the last drive I bought) has a ~9ms seek time.
Access time = seek time + rotation time ( + negligible other times, well under 10~100us)
Rotation time ~4ms (3ms for 10krpm)
Seek time ~9ms (supposedly ~4 for really high end drives)
Now this is just with the technology we have right now. I do not know enough about the constraints on hardware to know if they could push the limits. For instance, if the actual write (/read?) bandwidth could theoretically be higher, would it be possible to just rotate the disk faster and/or bump up the head seek time?