If I choose to export audio at 96k in Cubase (not a realtime export), does it render my project at 96k, or does it render it at my project sample rate (48k) then upsample?
The upsampling is done while rendering.
Sorry I should have clarified, does it mean my VSTs are running at 96k or are they being output at 48k and then being upsampled?
As far as I understand it, your original sample rate is kept, just the upsampling is done within the export process/doesn’t require an extra step. But it’s no ‘true’ higher rate.
OK cool, thank for clearing that up. I can hear mixes i’m bouncing at 96k 24bit sounding better compared to their 48k 24bit equivalents when both are rendered to 44.1k 16bit. It’s subtle, but it just seems to be a little more open with a touch less harshness.
I know it’s not advised to up sample but I think i’ll keep bouncing mixes at 96k then running the related master projects at 96k so it’s plugins can operate at the higher sample rate and see if that makes it ever better. Hopefully the benefits of running a project/plugins at 96k outweigh the downside of up sampling the audio from 48k.
If it sounds subjectively better don’t doubt it, that simple 
Tried the exact same approach years ago by upsampling mixes before mastering (think I did it afterwards with Crystal Resampler in Wavelab). Have never been sure if it was ‘better’ in any regard - but - using a lot of UAD plugins some of which do internal upsampling for processing the difference might have been marginal or non existent anyway. Looking at it this way I seem to believe in the benefits of upsampling. Paid money for it 
Yeah whatever sounds ‘best’ is the one to go for regardless. 
Try doing a mixdown at both 48 and 96, load both audio tracks into a new project and reverse the phase on one track.
They should really null, meaning there should be no sound as all.
However your sound card may perform better or sound different at 96k and that’s what you are hearing.
If you want to benefit from plugins running at a higher sample rate, you would have to set the project to 96k.
There are more to it, like plugins that are not reporting the correct latency and therefore have smaller inaccuracies when run at 96k if the buffer size stays the same. All highly system dependent, you would have to do null tests to figure out what exactly makes the difference. Or just do what sounds best to you, if it sounds good is is good 
I just tried to null test even just a straight bounce of the mix at 48k and 96k, so not even going via the 96k mastering project stage I now, and it didn’t null, and the 96k was definitely better. It sounded more cohesive with a smoother less harsh top end, a bigger bottom end, and overall more transparency.
I’m definitely a convert to bouncing at 96k now!
I did try to convert the production/mix project to 96k but my computer couldn’t handle it. 96k in the mix project (which is also my writing/production project) just isn’t an option for me as I use too many ASIO intensive plugins.
Yeah, that was going to be something I would suggest. Years ago and I have no idea how many at this point, there was discussion about recording at 44.1 then up sample every audio file to gain access to higher sample rate processing during the mix stage. The resolution gets very additive at that point.
As a thing between I see stem mastering at higher resolutions if it’s worth the hassle to you. Just a monster computer will handle a 100+ project well in 96k I guess.
The UAD auto upsampling where-it-makes-sense is a great thing but you need the expensive hardware that would buy you a real monster computer on the other hand. You’d need twice as much storage as in 48k though.
Before anything else I’d do some more testing, comparing standard to higher sample rates. Setting up a project that takes everything else out of the equation (as soon as you have nonlinear processing going on a null test isn’t working).
Peakaes hint that your audio hardware might perform better at higher rates is good! In this case it’s just you hearing a benefit. Often read that some interfaces will even perform worse as the ability for higher rates is just build in for better marketing but far from perfect technically (each cheesy cheap interface has 96k today…).
The whole thing seems not so easy obviously. I left those freaky thoughts behind and do 44/24 all day long as it sounds really great to me