I too would emphatically agree with those who recommend avoiding the Doug Thorley exhaust. It is a noticeably loud exhaust and the one I had (first generation DT), suffered broken welds within a short time. It was discarded. I have also had a SOLO exhaust, quieter and well-built but for my taste, still a bit too loud. I currently have a SILA system and I am quite happy with it.
The Brabus-like body kit you are considering probably is intended to accommodate a Brabus exhaust opening. Some of the kits out there have a trim ring available which will allow you to utilize other exhausts with various opening requirements and positions and keep things looking clean. You may well have to perform some surgery on your rear skirt, so make sure that you have your desired exhaust in place before any cutting is done prior to the subsequent painting.
As far as painting goes, try and get the skirts painted with the car there rather than just taking in the skirts by themselves. A good painter will be able to more closely match the color to the car, if needed. I had my KuhlFx package painted without the car being present and the smart white painting done by color code only does not exactly match the car color. It’s not different enough to get me excited, but I do notice it and have seen this type of mismatch on a few other smarts with ground-effect kits.
A few warnings about ground-effect packages in general:
You will probably see reduced levels of ground clearance with your kit, this meaning that you should begin viewing moderate-to-steep driveways as the enemy, requiring angled approaches and exits. Also, you may want to invest in a low-profile jack to access your side jacking points as they are usually difficult to ge to with the skirts in place. I recently went to an independent Mercedes shop that also works on smarts and was refused service because the owner felt he could not safely place my car on his lift system without damaging the skirts. In the future when I need service I will have to remove the side skirts before going to the shop. This is just another pain-in-the-posterior to be put up with.
In conclusion, with all of the considerations associated with this type of customization, I’m pleased with the visual change and would readily do it again. Possibly in 2014, when the next generation of smarts+Renault show up, I’ll be doing it again. Good luck