SADX Steam by no means is just a hook into Steam unless it's being compared to the Dreamcast Collection version, in which case you would be right, UltimaXtreme. However, there are some rather large differences between the original (moddable) and Steam versions of SADX. Specifically, one sucks, whereas the other does not.
Character model swaps work, texture modifications work, but that's where it stops for the half-baked Steam version. It's a port of the 360 version which is a port of the PC version which is a port of the GC version which is a remake of a revision or another of SA1.
It has a low internal resolution (meaning it's running at 640x480 no matter what you set it to) which dare I say is unhackable by any conventional means, especially when it comes to removing the borders for widescreen (I've tried myself). It hasn't been disassembled and so most of the good mods are based off of the original which has. Just using the executable from a mod of the original in the Steam version will work for the most part, but audio and such is formatted differently and isn't even in the same place.
At the end of the day, you're not modding your game, you're getting resources that were from the older version that were modded for the older version. The only thing this Steam version has over the old one is that it uses Xinput (specifically for 360 controllers; no benefit otherwise), it uses DirectX 9 as opposed to 8, and uses looping ADX audio instead of... *cringe* WMA.
They also removed some types of Chao. =|
