Hi
SSD provides faster access speed means programs can run more quickly, which is very significant, especially for programs that access large amounts of data often like your operating system.
And for Upgrade related information on this unit, You may refer to below document.
Refer to SSD and CPU section for the supported and tested upgrade on this unit.
HP Z800 Workstation:
http://www8.hp.com/h20195/v2/GetPDF.aspx/c04117130.pdf
I hope that answers your question. 
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