The 1541 Ultimate isn't a 'surrogate processor'. It emulates a disk drive and a tape recorder, which is accessed in the same way that the real devices would be in a cycle-exact simulation, which means that they are compatible with turbo loaders.
As for the extra memory - that acts like a Ram Expansion Unit (REU) that was produced during the commercial life of the machine, only expanded to 16MB in the same way that other clones were. Honestly it's more like a RAM disk than a memory expansion, so not even as flexible as an Atari memory expansion. You need to copy in and out of the expansion RAM through a port and it stops the CPU whilst this is happening. You do get the advantage of a 1 byte per cycle transfer (which is impossible using a 6502 loop) but the downside is that it has to be a linear transfer so it's not easily abusable as a blitter. You can stop destination address incrementing however so it will hammer a single address with a sequence of bytes once per cpu cycle.
It adds nothing to the CPU power of the machine. That remains the same as ever.
The one awesome thing it does do that wasn't generally possible back in the day (unless you were a studio with an expensive PDS setup)is allow you to inject and run code into RAM over ethernet for cross-compiling and hardware testing, and if somebody ever points me towards an Atari 8-bit device that does the same thing you can be damn sure I'm having one of those!