I seriously doubt there is a ground loop in the pedal. To be sure, grounding is a tricky thing and what works in one situation may not work well in another. There is a concept known as "single-point grounding." Without getting into the theory, the idea is to control where the ground return currents flow. With a guitar pedal, the currents are tiny, the frequencies are low and the box dimensions are small. For all practical purposes, the entire pedal can be thought of as one single-point ground. What that means is you can ground everything that should be grounded any place you like inside the box and it won't make any difference, as long as they all connect together somehow. I have never had a problem with a pedal that was caused by too many internal ground connections. Power amplifiers are a different story because of the larger dimensions, higher currents and stray EM fields from the transformers.
The SFTii pedal is more sensitive to 60Hz (50Hz if you live in a 3rd-world country like England) pickup because it has very high gain. Scraping paint off of the mating surfaces between the case & bottom cover will make the Faraday Cage more effective.