Data Digest Checksums do have an overhead, mostly on CPU cycles. From looking at some internal notes - it can be as little as ~5% overhead up to 15-25% - the latter being very large sequential workloads. Of course your milage may vary.
Yes, you can enable and disable it on the fly. The Nimble Connection Service (embedded within Nimble Connection Manager) maintains the relationship and availability behind the scenes.
There is an iSCSI Digest FAQ available on Infosight which will assist in your ultimate question - should I or shouldn't I use it.