I picked up a Behringer USB mixer last year to try as an interface. It's on the shelf - too noisy ...

What do you have in the preset in GR5? If you have any amp/preamp models loaded, the noise is expected because you'd be feeding the output of one amp into the input of another.

I mostly feed signals into the computer for recording rather than back out to an amp, but I have fed signals back out to my Laney IRT Studio amp without any noise problems.

Some effects are best fed into an amp's guitar input; pre-preamp. Other are best applied in the effects loop. IMX, the RP1000 or GT-100 work well with a regular amp because they can be cabled (4-wire method) to insert the different effects at different points; frontend vs effects loop.

I traded in my RP1000 to get a GT-001 desktop unit, which I also like a lot for studio use.

This isn't hard and fast, but is a good starting point figuring out where to insert effects: