Conversion rate massively depends on how well it is presold. I have some affiliates with <1% conversion, and some with >5%. Some people are quick to drop a product for bad conversions, when a lot of the time it is there own poor pre-sell which is causing the crap numbers. So yeah, you'll have to work out your own, and tweak and test.
There is actually a way to link directly to the Clickbank order page skipping the vendors buy page. I saw a youtube video on it recently but you'll have to search. If their buy page is shit, its a good thing to do