When I tested the UC 2 years ago I built a test in Reaper and Logic that used instances of NI Reaktor's "Space Drone". I did not count the number of instances, but just made sure to load the CPU close to 100%.
Then I played and recorded through *all* I/Os of the UC at 48 samples on Windows and 32 samples on Logic without dropouts. This test was meant to prove that the RME USB driver/protocol is not a culprit.
Space Drone creates artificial noise and tones, it doesn't need much bandwidth like playing lots of samples through Kontakt, it just eats CPU cycles (and likely may even fit into the CPU cache). I had to trick Reaper to real-time CPU priority, but that's not the RME driver's fault.
The UFX uses the very same USB driver (maybe with some specifics for the UFX) and other than the higher number of I/Os I don't see why it should not work the same. In most real-life situations you won't get close to anything higher than 65% at low buffer setting, likely not even 50%. That is because most audio software and hardware won't allow that.
The drawback of current quad-core CPUs is that they start at a rather low clock-rate (2.3 on mine). And while they will use at least a single TurboBoost blib even when using all cores (3.1 gHz on mine) they will lower clock-rate when temperature increases. Running those 40 tracks (+reverb returns) in the Ableton Live test made my CPU run around 2.5 - 2.8 gHz. Since these values can change depending on fan-speed, ambient temperature and load you never know what your real maximum load is. On Windows you can turn off TurboBoost, but that leaves you stuck at 2.3 gHz (in my case).
That being said, for FX using 4x 2.3 gHz on an i7 (with a prospect to run up to 3.2) is still better than using 2x 2.8 on a Core2Duo.
Last but not least: The most compelling reasons to use a Macbook are either because you need OS X software or because you want to use it on-stage knowing that thousands of other musicians use the same computer. Other than that it might be worth taking a look at a specialized builder like Daniel suggested.