This is probably a no from the start as the decision falls on the forum owners. They won't do anything that "endangers" their income in any way. That's why I paid for Premium as well, to put in my signature idea. I was rattled at first because of the size limitation, but I came to terms.
But as a suggestion to this rule, I would like the size of the signature to be increased to 800x400. It's not such a ridiculous jump IMO, but I may be too subjective in wanting this.