When you have Finished tier 5 what is the rom mark bonus you get?

nothing. prior to the changes made, the upgrade project you have access to change marks to dil would have unlocked after tier 5. however, due to massive outrage, they dropped the requirement altogether. there is no bonus anymore per se.

I thought you got some bonus like you do with the omega rep

Like Omega, you get 500 marks, and (if I recall) 32,000 Dilithium Ore. Unlike Omega Rep, however, you have to complete the tier 5 mission rather than just hail Admiral D'Vak to get it.