Only in the scope of racing will 8 ounces be significant, so if the question asked came from a pro racer then yes you are correct, it is significant, after all, you're racing for money, but I seriously doubt a racer would come on here debating between 105 and Ultegra, so the poster probably was not even a Cat 3 racer...although I knew quite a few Cat 3 racers that were using 105 but I digress; but in this case he's not a racer so it's irrelevant, besides if the poster wanted to go faster he would have a more dramatic effect spending the money he would have saved going with 105 vs Ultegra and upgraded the wheels instead.

He could, depending on his current body weight to height ratio, lose some body weight and end up being faster on the bike, for cheaper than Ultegra upgrade.