Calibre works MUCH better with a T1 then a Kindle Touch. With the T1, you can have Caibre put books on the T1 and in Collections. You cannot do that with a Kindle Touch.
The T1 supports multiple fonts. The Kindle Touch doesn't unless it's some embedded font in KF8 in which case you get a LARGE line space because it's a bug that Amazon seems not to want to fix.
Yes, the Kindle Touch has large line spacing for KF8 (AZW3) eBooks. So a lot of the new eBooks will be downloaded as KF8 and you'll have this line spacing that's just not that nice.