If you are going for low noise, then the Silverstone will be better, as it is semi-passive and won't run the fan at low power draw.
@Aibohphobia is at Computex right now, so he is probably busy going to meet various vendors, writing up content, etc. I haven't disassembled the SX800, so I can't give advice with hands-on experience for substantiation, but if you are just concerned the shield won't be able to wrap around the fan, as it is thicker, I'll remind you that you are modding it all in the first place, so you are free to make creative solutions (or not-so-creative, if you please, like duct tape, that just works) to remedy these issues.
I'll be honest, I am not worried about the shroud since I can figure something out like you said with modding. But it is that reflector tape "non-contact tachometer" that has me on edge since I've never knew that was a thing in PSU's. And the main reason I am trying to do this is mainly aesthetics for a theme. I noticed the Corsair fan's rpm is low ( at 1650rpm ) compared to the stock fan which is nearly 2300rpm so that is a secondary concern if this mod is even possible.
Also, since the Corsair fan is 10mm thickerI will be cutting into the shroud or may just mount the fan on the outside of the psu without the fan grill.