The fastest CPUs you can upgrade to are 2 x 3.46GHz 6-core chips. The definitive list of compatible CPU upgrades is here - Mac Pro CPU Compatibility List | MacRumors Forums
Having previously upgraded my own MacPro 2010 from 2 x 2.66GHz 4-core chips to 2 x 3.06GHz 6-core chips I can tell you that you will get a couple of additional bonuses. First if you have RAM that supports 1333MHz your Mac will start using the RAM at that speed instead of your current 1066MHz, secondly the inter CPU connection speed will also increase to 6.4GHz.
The fastest SSD you can use with a MacPro5,1 is an AHCI PCI based SSD and would reach nearly 1500MBps. You cannot boot from newer NVMe type SSD drives but could potentially use one as a data drive. An alternative is to use multiple AHCI SSD drives in a RAID0 (stripe) array to get even more performance. See - http://barefeats.com/hard210.html
PS. Another way of getting a small speed boost is to ensure your RAM is configured in triple banks, that is 3 x 8 or 6 x 8 is faster than 4 x 8 or 8 x 8.