Upgrade Readynas Pro 6

If the bios upgrade is successful, it should allow you to use the e7600 which is definitely much faster than the e2220 or the current e2160.  And don't forget that these units can actually take up to 8gb of ram so someday when those modules are cheap enough, you can upgrade to them and continue to extend the unit's life.

These units were made really well and since a lot of the components are standard, they can be upgraded and live on.  I actually am in a similar situation as you as I purchased an Ultra 6 fairly cheap with the intent of running a few spare drives in it since it has 6 bays.  It is also potentially a better use case than a WD EX unit I have off site as this unit has dual lan connections which I need at the site where the WD unit is.

I have always had a soft spot for working hardware.  If it still works, it deserves to live on until its demise.  The hard part sometimes is finding an application since a lot of older hardware is simply destined for running things only from its era.   Luckily, nas units don't have as much of an issue with this.  I actually have 3x Intel ss4200-e units that I've restored.  Ironically, they are also using standard lga775 processors.  I even have an older synology.  And the great thing about multiple nas units which are all different is that they will never all fail in the same way.  So besides the normal best practice of 3-2-1 backups, having multiple nas units in the mix adds even more redundancy, especially when off site.

One thing I would do if you haven't already is to test what type of transfer speeds you are currently getting from the nas with a single drive.  This can be your benchmark so you can see how much of an improvement you get from the cpu upgrade.  I would also benchmark both os6 vs os4 since you have to downgrade to os4 anyways.  I don't know if that's ever been done before either.

I've found that all of my nas units are in the same range of speeds with the synology and the readynas towards the top, but these units also have the newest/fastest drives in them, so it could be just drives.  The exception to that is the intel units as even with a full ssd array, there was no improvement to the maximum transfer speeds.  That's actually the long term plan with all these nas units--as ssds become cheap, replace drives with ssds and then these units will become even faster than they are.  There's no reason why these can't be working 20 years from now.