First, I think you mean 7.3.5.1P2. Second, there are a number of changes that will make the upgrade easier and any reversion back to a 7.x (7.3.5.1) much easier incase the upgrade to 8.0.1 goes wrong.
You can easy view the bugs fixed between 7.3.1 and 7.3.5.1P2 on the NOW site.
Follow NetApp's advice and your upgrade will be a lot less painful. Since this is a major version upgrade, it is important to get it right the first time. Reverting back to the previous release is a major PITA.