There's one very important aspect I think you are missing in your assessment:
How do you plan to recover?
When raid5 loses a drive, it will run in a degraded state until it has recovered automatically. (At least if you have a hot spare at hand.)
When a raid0 loses a drive, it cannot ever recover at all. This means you have lost redundancy, and to recover it, you need to rebuild your raid0, and copy all the data (not just the data on the broken drive) back from the secondary that is now under production load. That is, instead of the single degraded raid5 array, it's now your entire production setup that gets the performance hit.
If the raid5 (or raid6) degraded state performance penalty isn't something you can cope with, you should probably do raid 1+0 instead. Yes, it costs more, but disk prices being what they are, it's going to be money well spent.
Maybe "actively monitor the raid5 state, and transfer the load off the primary when a drive fails" is the solution that gives you most of the benefits without any drawbacks? (Apart from losing the coolness factor of running without any local redundancy, of course.) If your raid5 drive recovery is taking a lot longer than a complete database data sync, either your raid software is acting strangely, or you have seriously oversized disks, I'd think.
