Maybe get your point, but reading lost of relevant posts and they are starting to become unusable mush.
Are you stating that Latency is not the issue here ? The why would RME offer more than Clarett 2Pre USB?
I haven't A/B-ed the two, but all things being equal, higher-end interfaces simply sound better. They have better electronics surrounding the digital converter chips, and if they're not designed better, they cut fewer corners.
Or at least you'd expect that to be the case.
RME makes high-quality interfaces. The guy who does their engineering, I believe the head of the company, is really good. They've been able to boast excellent latency specs, and their drivers are solid. That was especially important in the early days of software sampling - their cards were the ones most people preferred for Gigastudio machines.
Now, I don't mean to say that latency isn't important, but as people have said, the big fish is how powerful your computer system is, and especially whether you have SSDs. A more powerful computer will let you work at a lower buffer setting; each setting usually increments up or down by 100% (128 vs. 256 samples for example), and that's pretty drastic compared to the driver (because all the drivers are fine these days).
Plus you're going from the keyboard over MIDI to the software instrument though your DAW to the output. And latency with a slow-attack sound is all but irrelevant; it only matters with sharp-attack sounds. Also, some people are more sensitive to latency than others.
The main thing is that the first thing a lot of musicians ask about is the latency spec. That's what marketing will do to people's brains.
What real men and women ask about is how good the interface sounds and what its features are - like how many ins and outs it has, how it connects (Firewire/USB/internal), what its monitor control features are, whether its mic preamps are good, what kinds of inputs and outputs it has...
Speaking of my favorite subject (myself), about 12 years ago I had several interfaces here for a big article. The RME Fireface was the third most expensive and it sounded the third best. But the differences at (then) the $1500 - $2000 level were extremely subtle.
I ended up choosing the Metric Halo 2882, which still kicks serious arse day in and out - and they just came out with a major hardware update for it that lets it connect by Ethernet, among other things. But its mic preamps don't have a lot of gain - I use a stand-alone one made by Millennia Media.
The other choice was the Apogee Ensemble, which sounded fly poop better (and was more expensive) but has since been abandoned. So I made the right choice.