We do know that on average the duration of government is shorter under more proportional systems, which seems to suggest that they are more likely to have, at some point, somebody pulling the plug.
In respect of policy outcomes, however, it might not necessarily be that different from what we see now. I want to emphasize that there's little evidence that small parties carry more weight in policy decisions within the government. There is, as far as I know, little empirical evidence for that.