Take a look at the blog post on http://d.hatena.ne.jp/hiiragi-june/20080528
意外に feels more like written Japanese and 意外と is more colloquial. They
both grammatically correct and they both have the same meaning.
Although 意外に feels more traditional.
The rest of the blog post is about history of dictionaries and some dictionaries actually don't even talk about the difference between these two.
According to http://dictionary.goo.ne.jp/leaf/jn2/10029/m0u/%E6%84%8F%E5%A4%96/
Currently, in the same way as「意外に」, 「意外と」has been used like 「意外と知られていない事実」
So to answer your question, yes they are pretty much interchangeable. Note that に and と are generally not always interchangeable, but in this special case it is.
Also note that the meaning of 「意外に」and 「意外と」has more to do with something not matching one's expectations and expressing one's surprise, so it is more accurate to translate it as "unexpectedly", rather than "more so than I thought", although in some cases it means "more so than I thought". I know I'm being nit-picky here but just want to make sure you understood it right.