As far as I know, MX5 *are* MX80 with a license limitation that grant you the usage of only 1 MIC slot.
Apart from this limitation, the general performance is the performance of a MX80 (55 tp 60Mpps in the datasheet).
See here for an old datasheet with interesting figures :
http://fr.security.westcon.com/documents/41441/juniper%20Datasheet%20MX5-10-40-80%20.pdf
For RIB and FIB :
55 Mpps
1,000,000 IPv4 prefixes in the Forwarding Information Base (FIB)
4,000,000 IPv4 prefixes in the Routing Information Base (RIB)
16,000 logical interfaces (IFLs)
512,000 MAC addresses
https://www.safaribooksonline.com/library/view/juniper-mx-series/9781449358143/ch01s02.html
All in all, good boxes for forwarding, but their Routing Engine is said to be slow. So if fast convergence with very large BGP tables is a concern, maybe you should look elsewhere. With small to medium routing tables, there're very good boxes.
