It's not that so much, but that's a factor for sure. I think Sport mode is doing everything right for top level racing. There are a few things that could make it more interesting, but ultimately it should be down to who's fastest.
To shake up the field in less formal events I'd propose some sort of grid randomization and/or...
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Fixed tuning should definitely be in the cards across all levels of event, particularly manufacturers cup - where PD needs to sort the cars well - or any race with a fixed and limited list of allowable vehicles (and obviously shuffle race.)
The only thing you could tune in GT6's quick match loaner cars was brake bias, and that made for some awesome racing. There were plenty of events with tuning and they were awesome too but you had to really do your homework. The tuning factor really bugs me as someone without the time or interest to squeeze out tenths with tinkering. It was especially discouraging when I'd pick a car I like, get it up to PP limit, give it a quick tune I felt happy with and then I'm 8 seconds off pace... on TSUKUBA!