1. Although the weather capabilities are neat do they detract in any
way of the performance (transmission/reception) of the unit?

No

2. I have room for a full size unit in my pickup but I may put a unit
in my explorer too where size is limited. Are the uniden compact radios
any good?

Yes

3. If I'd like to get 20-30 miles range (assuming a mod) is any of
the unidens better to mod than others.

Big question mark here. Noise, atmospheric and man made determine
your distance. At night when it's whisper quiet you can easily talk 20
- 30 miles.