Some thoughts on tires for the "F": Of course the Lexus engineers during all of their testing chose a tire that they felt best suited the IS-F. As a matter of fact the Michelin was built specifically for the F if my memory serves me correct. (Any input from other F champs accepted here!) The tires that come on most high performance cars these days, especially in 19" are of a grippy compound that give pretty stellar handling. However, for that grip you give up treadwear...So the goal would be to find a good compromise in decent treadwear and good handling and braking performance. Toyo tire has a pretty decent tire that fits that bill in the T1-R. It is offered in the factory sizes as well as some plus sizes. (for the fuller size 
) We use Toyo on almost all of our project cars because of the good wear characteristics, good looks and performance. BTW, factory IS-F tire size 225/40R19 93Y front and 255/35R19 96 Y rear.
As far as the XL load ratings go, you can also look at tires that are designated as RD or reinforced. They are classed with the XL tires:
Quote from Toyo:
Standard Load vs. Reinforced (RD) or Extra Load (XL)
‘Reinforced’ and ‘Extra Load’ both refer to the tire’s ability to carry
additional load capacity at a higher inflation pressure compared to
standard load tires. The sidewall of the tire is marked with either
“REINFORCED” or “EXTRA LOAD”.
If these markings are absent on the sidewall, then it is inferred that it
is a standard load tire.
The Toyo T1-R in the 19" size for the "F" are RD.
http://www.toyo.com/docs/tires/tires.asp?lpid=18966&name=Proxes%20T1R%99&category=sport#
) We use Toyo on almost all of our project cars because of the good wear characteristics, good looks and performance. BTW, factory IS-F tire size 225/40R19 93Y front and 255/35R19 96 Y rear. As far as the XL load ratings go, you can also look at tires that are designated as RD or reinforced. They are classed with the XL tires:
Quote from Toyo:
Standard Load vs. Reinforced (RD) or Extra Load (XL)
‘Reinforced’ and ‘Extra Load’ both refer to the tire’s ability to carry
additional load capacity at a higher inflation pressure compared to
standard load tires. The sidewall of the tire is marked with either
“REINFORCED” or “EXTRA LOAD”.
If these markings are absent on the sidewall, then it is inferred that it
is a standard load tire.
The Toyo T1-R in the 19" size for the "F" are RD.
http://www.toyo.com/docs/tires/tires.asp?lpid=18966&name=Proxes%20T1R%99&category=sport#
