I don't believe STR accounts for only 5 percent of the overall performance of a drive, I think it has much greater influence. In the end however, it's the whole package - STR, access times, command queuing optimizations, caching strategies, even interface transfer rate - which leads to a certain performance level. If there is unbalanced performance in on these low level characteristics, a drive will not reach its expected performance level.
In this case, the problem is probably with the caching strategies. The 15K.4 and 15K.5 seem to be totally de-optimized for workstation usage. If the 15K.5 has to perform a lot more seeks than other drives with proper caching strategies, it will still be behind in performance, despite its stunning sequential transfer rates. That doesn't mean higher STR won't help. Compare the performance of the 15K.5 to the 15K.4 and you will see the 15K.5 performs substantially better in four out of five of SR's RankDisk benchmarks. Since both drives are poor in caching and access times are about equal, the difference is probably caused by the higher STR of the 15K.5.