what do i need to fit 20's to my tf sedan do they need to be of a certain offset ?

It is not the diameter that you have to worry about. It is the width of the tyres unless you go for the drift look and have wider wheels than tyres. With bigger diameter wheels, the profile of the tyres will be lower to maintain the overall diameter to within specifications. Need to check with RTA how many millimetres you can increase or decrease the overall diameter by. To calculate the overall diameter do as follows:


Thickness of tyre section = Width of tyre x profile of tyre / 100
Overall diameter = 2 x thickness of tyre section + diameter of wheel in millimetres


So, for a 20x8" wheel with 245/30R20 tyres, the overall diameter is:
Thickness of tyre section = 245 x 30 / 100 = 73.5mm
Diameter of wheel in millimetres = 20 x 25.4 = 508mm
overall diameter = 2 x 73.5 + 508 = 655mm


Your TF originally came with 205/65R15 tyres on 15" wheels with overall diameter of 647.5mm. That is only 7.5mm smaller in diameter or 3.75mm smaller in radius so no issues with fouling wheel arch and next to no difference with speedo. However, 245 tyres are 40mm wider than 205 tyres so that means they will stick out 20mm either side of the centre line. If there is way more than 20mm between the inside of the tyre and suspension components than the offset can remain as 46+. If however, there is not much room then you need to reduce the offset down to something like 38+. Note that legally you cannot change the offset my more then 12.5mm as you cannot change the track by more than 25mm. The other thing to note is that if you reduce the offset too much than you are likely to have the tyres sticking out too much and having the tread visible is also illegal.


Thus with the above tyre size with 46+ offset, the tyres will be 20mm further in and stick out 20mm. I reckon with 245 wide tyres, the lowest offset should be +40 so with +40 offset, it will stick in 16mm and out 26mm.


The following is a very useful site to compare tyres and wheel sizes:
http://www.rimsntires.com/specspro.jsp