what springs???
what springs???
I can't see there would be enough of a difference to worry about really.
The main suspension components are the same, but the IS300 has different part numbers for its springs. There would be a more significant weight difference there though, i.e. 3.0 vs 2.0 engines.
Here are the specs for Eibach springs. The TTE ones are rebranded eibachs.
Can't find any details for Altezza's. If you are getting an IS specific ones i'd get the Is200's as it'll be the closest match.
Ed