Installing FTX Global Base is most likely the best single think you can do to enhance the look of your terrain. It really replaces the default textures with improved and optimized textures. Some report in the forums that Global base actually made the sim run better. I have not had performance issues with Global base on its own.
I don’t recommend Global Vector as it makes for lots of problems and cut deep into VAS.
From the base pack, any scenery you add will have some impact on performance. I find the land classes are not too bad on their own - very livable.
That said, it all about compromise with sliders when you start adding to the sim and finding where its going to run the best takes some testing.
Hop that helps