One thing you should realize is that you have a significantly different beer with that much difference in OG. It's probably a good bet it will also turn out drier than normal as well. For this reason, your bitterness may seem much higher than normal. I learned this with brewing German pilsners where you are striving for a very dry finish. The required bitterness ratio is actually much lower in this kind of a brew than you would expect for the apparent bitterness you taste. The FG and dryness of the beer play a big part in this. So while normally 5 IBU difference is not noticeably different, everything else being equal, in your case everything else is not equal.