I'm sure you could as almost everything is customizable. We just happened upon Flat Awesome and it was a good fit for an upgrade from our 2.5 year old style.
One thing to consider. I think paid/premium styles are probably better supported in the long term by the style designers. I wish I could throw extra money at @Russ and have paid for branding removal as a token of support.
There are a few really nice premium themes here by some top notch devs. I really don't think you can pick a bad one. Choose what you like.
FlatAwesome is a new style with a few customization options.
UI.X is a theme framework with a ton of actual added features in it. Stuff like sticky scrolling or collapsible sidebars or other functions that are not part of XenForo's base functionality, and the ability to change just about everything you could want.
If you like the way FlatAwesome looks and you think all you'd like to do is tweak colors or fonts or whatever and use it, it's probably a great way to go. If you'd like a full framework with the ability to customize tweak and add features to your XenForo install, UI.X is a better option.
I installed a few themes (FlatAwesome being one of them) and was never very happy with the amount of customization. That's not a dig on the themes, they were all excellent, just not exactly what I was looking for. Having the full suite of customization and added features in UI.X is pretty awesome. I think there are a couple other theme frameworks with full customization, I just haven't tried them.
Since you're asking about heavy loading... Flat Awesome does not have the bells and whistles of a framework, whether it's our framework or UI.X. Flat Awesome does excel in the area you're asking about it... load time.
It removes all gradients, all references to images(aside from avatars obviously ) and no added JS for those bells and whistles. Plus it's free, give it a go and if you don't like it move onto the next one .
So in regards to heavy loading for UI.X, definitely no more than any other theme. But if you want the advanced features now or in the future, UI.X is the best of both worlds.
"The simplicity is the maximum sophistication"
If you've decided to install a theme, find something that give you exactly what you need or exactly you will use.
Instead buy something with tons of feaures that in almost all the cases you will never use and probably will give you headaches in future updates of Xenforo.