I don't know, I think the interior, cabin quietness, and driving comfort in my Fiesta ST (with Recaros) is great. My mom's '14 Focus SE interior doesn't seem any better. I was never in a Focus ST long enough to really notice any differences in interior quality, but I have been in a FiST and FoST back to back and didn't feel like the interior quality was drastically different. I came from a 2003 Ford Escort ZX2 with stiffer springs that handled worse than the Fiesta with a seat that really wasn't that great, the interior that always rattled and was falling apart, and the loudest cabin ever on the highway.
So that being said, it's really all in perception. Personally I think comparing the 2003 Escort ZX2 to the 2016 Fiesta, both the cheapest cars in Ford's lineup in each particular model year, the Fiesta is leaps and bounds better and I am more than happy with the Fiestas interior for the price point. The extra money I paid over the base price of the Fiesta SE for the ST was for the engine, trans, turbo, Recaros, orange paint, Sync 3 with nav, keyless, ST body pieces, suspension, wheels and tires, and anything else a base ST adds on, not to mention the engineering that goes into making the ST an ST. All that stuff adds up to a pretty dang good value for me. I can't think of any other car I'd really enjoy having right now for the price.