Upgrade Controllers= Nope, there are no upgrades from Apple to do this.
Upgrade Drives= Possibly but will be hard to find. The X-raid only uses PATA drives which are hard to come by and the largest we found recently were 500GBs. 750s which were the largest are no longer being made not sure you will even 500s we looked hard and finally found some last year.
Your best bet is purchase a new SATA or SAS drive Array. We bought a Sans Digital 8bay storage with 1.5 TB drives with RAID5 via Highpoint Card. 12TB RAID5 machine for under $2,000 that’s 2x as fast as our X-Serve RAID.