In general, I find that partner search sites lead to good conversion rates. Depending on the product, some are much higher (and some are a little lower) than Gs main site.
The main reason I won't have an AdGroup's ads immediately syndicated to partner search sites, is if I'm concerned about the keywords/ads meeting the minimum CTR as they might be too general. In these cases, I want to see the exact CTR on Gs site alone, as thats the number that mattes for keyword status. Once I've gotten a decent history and handle on those general keywords, and they've proven profitible, then I'll have the ad appear on search partner sites as well.