Hi there
I think the whole point of the GR is that Toyota made a homologation special, or however you spell it and a car with its intended purpose being rallying which can mean tarmac, gravel, wet, snow, bumpy type surfaces the car has to be able to work well in all those conditions which also generally sum up a British road and our weather perfectly.
Its not been designed to be a race/track car, albeit the wording "circuit pack" can be seen as miss leading. The none circuit pack car is a soft car, with fantastic damping and ride quality, but equally lots of lean. The Circuit Pack car does not turn into a harsh car, the dampening is still very good, the fact these cars weigh so little helps of course too, but I believe the Circuit Pack cars just have slightly revised dampers/springs, thicker anti-roll bars and of course the very important torsen differentials.
I feel Toyota made a car that was all about feel or at least the best they could do in this modern age, so its all about mechanical grip and feel, yes it does have electronics but the torsen differentials are mechanical and very feel some, the suspension is all passive/fixed so again aids how you can feel the car but its a car you can use.
Seating position yep its high, if you want that low strung seating position then buy a sports car, this is not a sports car, its a hot hatch and pretty much all of them feel like your sat high compared to a proper sports/super car, but I actually take no dislike this, but having owned a Type R, Clio 172 and Abarth the latter two had very high seating positions and I was never bothered by it, so the Yaris is fine for me.
As to seat itself, I think its superb, driven the car now for like 4-6hrs straight and my back never aches and I always feel comfortable and cool, the bolstering is tame but as the material is cloth type I don't slide or move around in the seat.
I think as a B road blaster which seems to be exactly what Toyota designed the car to be in all weather conditions it is pretty much unbeatable.
If you want a full out track car I think the Yaris is maybe a bad place to start, Honda do that better out the box or just buy a sports car. I am sure if you further uprate those anti-roll bars, fit coilovers and turn them upto 11, add sticky wider rubber and increase the power then the Yaris would no doubt be an insane track car.
I think Toyota probably poorly named the UK version "Circuit Pack" as in different countries such as Australia the car is actually called the "Rally Version" which seems far more appropriate. Maybe Toyota should have used the word Rally here instead as one would expect a circuit pack car to have stickier rubber and even firmer suspension and be flatter, so maybe's Toyota's naming is a little miss leading or confusing to some.
If I was tracking the car, I'd get some 245-265 section cup 2 tyres on it with wider wheels to suit, coilover suspension, thicker roll bars and push the power to 300-350 levels and I suspect it would then be a devastating track car for its power class.
I see a lot of people with super car stables who have got these and some owners absolutely love them and are keeping them, particular a lot of GT3 owners and I also see quite a few who say the car is not as expected and are selling, or maybe their intentions was to flip the car all along.