For me at least, with the optimized loader library, yes, the impact of the DDS conversion is almost unnoticeable. However, I have a pretty fast CPU so I don't want to assume that it'd be the case for everyone, and the DDS conversion was done first, so even if it is overkill it costs me nothing to leave in and will better serve anyone with a slow CPU where mango and Pixman aren't enough.

Shortly after I released my tool, I had someone report that it was crashing for them because they were using a third gen Intel CPU that didn't have a SIMD instruction set the x64 command line portion used (particularly the BMI instruction set.) It was a bug in the mango image library anyway because I had disabled that particular instruction set when I built it, but goes to show that when you're doing a retro game hacking project, a lot of gamers are keen to use older hardware for them and I'm quite aware of this fact