If you can afford it get some products that remove compliance in the suspension (anyone know where to get them from?!)
FLOL
blatant plug is blatant
Personally I think all competition builds should start from the same point:
Safety first - you are the most important part to consider when you build a car so get the best safety gear you can afford. Get a decent roll cage professionally fitted, with the best seat and harness you can afford and get the mounts done properly because a good seat is no good if it rips out of the floor if you crash. Get an extinguisher - it could literally save your skin.
If you have a small budget stick to suspension and brake mods - this is what will make the car quick. A half decent coilover kit set up properly with a few sets of springs to try in testing and an adjustable rear bar will get you a long way into a quick car. Testing set ups though is where you will gain most time. Do tyre temp testing and change camber accordingly. Only change one thing at a time so you don't end up going in circles.
All good points though, being quick in a race car is all about corner speed without a doubt.
That said though, Trackday cars and race cars have slightly different requirements a lot of the time.
For most trackdays these days in the UK where you are typically NOT allowed to overtake mid bend, and hence often not able to go at your max corner speed on most bends anyway as you are limited by what other people are doing, its less cricual than it is in a race car, and likewise if you are allowed only to overtake in a straight line it means that you do need straightline pace to get past the people who are holding you up in the bends.
In a race you would overtake by outbreaking them into a bend but on a trackday you arent allowed to do so.
I used to have a trackday mini that very very quick in the bends, but lacked straight line pace, and it was epic fun outbreaking everyone and getting past them all in the corners UNTIL all the tracks changed the rules and said I wasnt allowed to overtake mid bend on a trackday anymore, at that point it became a frustrating car to drive, just following everyone else through the twisty bits and then being unable to get past them on the straight.
So I do think that people need to recognise the difference between what race cars and trackday cars each need to do.
Dont get me wrong, Im not saying cornering speed isnt good fun in a trackday car it is, but in terms of getting past the other people on the track its a lot less useful to rely on than it is in a race.
They are VERY different driving styles between a race and a trackday.