I tried it, and read striping is not the solution - as anticipated. DrivePool frequently reads from the slower, mechanical drive, even though all the data is also on an SSD.
Would you consider adding a feature "For mirrored data, always read from drive X" (or, in more complex cases, allow to specify drive priorities, such that DrivePool will always read from the drive with the highest priority among the ones that contain the requested file).
Should be reasonably simple to implement, shouldn't it?