7.5's really only offer an advantage at longer distances, IE further back on the trap field or doubles, where your second shot is further out.
The larger pellets carry more energy out further, but you have fewer of them.
At the 16 yard line, shooting singles, there should not be any appreciable difference between 7.5's and 8's but as a new shooter you are likely slow to get to the targets, thus they are further out and 8's have lost more energy than 7.5's would have at that distance.
That said, shot sizes and volumes really only make differences to the averages of the best shooters. Where that one extra pellet does break a target that really counts.
I've always found trap to be somewhat fickle, often you get new shooters out to the club, they try a round of skeet and a round of trap, and find they do way better at trap and they in turn tell you trap was much easier, I have two ideas on this experience.
One being that trap is most likely what you are familiar with at home, shooting hand thrown clays behind friends barn etc.
Secondly, it's a more natural shot....for that lack of better words.
But when those new shooters return, they find their trap scores dive, and it takes a long time to bring them up. They start measuring the targets and looking at the barrel and aiming it like a rifle, rather than in their first round or two when it was more instinctive and natural.
I've always been a skeet shooter, but I started out shooting trap, my experience mirrored the above. I've never been competitive in trap, but I can usually manage 22-24 without too much difficulty that said it took me years and years to get my first 25 in trap, where as I shot my first 25 in skeet within the first year.
The hardest thing about trap is the consistency it takes to win, running 100's after 100's without a miss. That IS hard.
Sorry for the ramble, hopefully you can glean a little something out of there,