Welcome to WebmasterWorld Loadesmoore,
We recently upped one account to 150% of the recommended budget (was running about 75% before) and have seen the cost per conversion actually drop about 30%. The products aren't seasonal so i have no idea why, but i'm not going to complain.
We have made ongoing changes to the campaign, but the big change was when we did the budget changes.
It all depends on your competition as well. If you are running a higher budget, you will stay on the page longer. So, if your competitors have spent their daily budget, you have one less ad to compete with, which in turn can lower the costs for you etc. This is one of the things that may have happened to us when we raised ours. Who knows? More testing required
Measure avg position vs convs and monitor cpc's against it all. Eventually you will see a pattern emerging.