1) So you mean we should implement MSDP encryption before creating MSDP pool & then there will not be any impact on de-dupe ratio
Yes, but it's not so much about before "creating" the MSDP pool, it's more about before "using" the MSDP pool.
But remember, if you do enable encryption at rest AFTER the MSDP pool has ingested backup data, then after the last of the unencrypted segments have been expunged, then there shouldn't be any difference with de-dupe ratios.
2) no overburned on media server
I do not understand "overburned". Please explain?
3) and no backup slowness issue ?
Apologies, I did not mean to imply that it "would" definitely go slower, just that it "might" or "might not" go slower for a while or forever. In truth, I don't know. If your server and environment and configuration are all top notch - then you may never notice any CPU or performance impact from enabling MSDP encryption at rest.
4) I have physical media server which has 2 octa core CPU , 2.6 GHZ , 256 GB RAM.
Sounds like it should be more than capable - but if the disk storage is slow, then MSDP disk IO will be slow.