My t5 grinds into 5th aswell.. Try shifting into first then quickly into 5th... Always do this!! 1st than reverse! May help... doesn't solve the problem.. but definetely eliminates some grinding..
Where did you get the t5... used?? I am probably going to be rebuilding my t5.. new syncro's etc.. you should consider having the t5 rebuilt.. if it is used.. can get a kit for about 250 ish on mustangs unlimited. T5 rebuild kit!! If it is supposed to be rebuilt already I would contact whoever you got it from....
The T5Z has a blocker on reverse, doesnt it? I believe so, which should help the engagement.
Anyhow, as Anthony said, put the trans into a forward gear first. This slows the cluster down so it engages Reverse easier. I would not shift quickly into 5th if you can help it. Any forward gear works - I often use 1st.
If that doesnt help...........