This seems to be rather like the sound issue, that some things have improved, some things not. While for me, these are the best bots in the GT series, they are something of a mixed bag. While much of my experience has been like HBR-Roadhog's, I've also blown through races with a 75PP handicap. The funny thing is, once again, the code for fast, competitive bots is in Arcade Mode. While they sometimes slow down and let you catch them, not always, and they rarely let you win. I've used Arcade races to train for tracks I don't know well or are new to, like the Kart Spaces and Ascari. I usually have to race my bum off, and occasionally I'm stuck a certain distance behind the leader and just can't get any closer.
Because of the multitude of cars and tunes available, this serves more than well enough as a difficulty setting, but I'm sure this hasn't occurred to a good portion of the fanbase who don't hang out at boards like this. Plus with rare standing starts, no qualifying and other factors which really would help out the game, I can understand people's vexations.
Maybe this is one of those things the coding team is working on. They're up to something, B-Spec isn't in yet, and they have a basis to work with. In GT5's Course Maker, every race was a standing start. There was qualifying in previous GT games, and in GT5's Online Mode. In GT5 Prologue, Arcade Mode had a wonderful difficulty scale of 1 to 100, though much of that was in assigning more or less powerful cars, but it's there. Arcade bot code in both GT5 and 6 are much more challenging.
Recode a number of events in Single Player to include this stuff, and GT6 takes a huge step towards 2.0.