Ah, now we're getting somewhere! Now that I understand the problem, it's quite interesting that there's no solution to it.
You see, because mATX boards allow two power-hungry cards to be used, manufacturers of such cases want to make every combination of components work in that case. This means, unfortunately, opting for a ATX PSU. Because Silverstone wanted similar CPU cooler compatibility with the SG01 and SG05, they had to make the SG01 taller in order to fit an ATX PSU and a <82mm cooler in there.
Now comes the interesting part: Silverstone has made a redesign of the SG05 called the SG13 which allows ATX PSUs to be used, but only with 61mm CPU coolers. Additionally, they increased the height by 5mm, not really noticable.
So the case you're searching for would more likely be an SG13 that's just two slots wider, or, as you said, an SG05 as wide as the SG01. Unfortunately, Silverstone already made those redesigns with the SG11 and the SG12 without reducing the height. This is most likely because they wanted to keep the 80mm GPU area exhaust fan at the back and compatibility with larger CPU coolers.
I searched the geizhals.eu catalogue for a case like the one you desire and what I found is: It doesn't exist. I looked at 155 mATX cases between 15L and 25L and none of them is what you want. The only thing that may work for you is the Silverstone GD02, a case that is long out of production and was intended as an HTPC case, but due to the dented top panel, modern GPUs with PEG connectors should fit, and it's the slimmest you can get with that setup. It does have its PSU to the side, though, so it's rather wide.
Everything else I saw was either a micro tower, only fits low-profile cards or only fits GPUs with their PEG connectors at the front, which is currently only done by the AMD R9 Nano.
I'm sorry to tell you that your only way of getting what you want of this seems to be to get an SG01/02/11/12 and mod it. Or you could get a micro tower and cut away the portion where the PSU normally resides.