I run both OS's, and any modern software that doesn't run the same on both OS"s is poorly coded period.
I have over 200 retail/paid examples to compare by just re-booting into either OS. Ableton, Reaper etc, hundreds of VST/AU everything CPU/RAM related is essentially a 1:1 match.
And Tutor is dead right on how 99% of apps are birthed in the coding/assembly stage. You're only going to see a "A Mac-centric design/execution of code" in apps like Drum Spillage and Numerology 3 and it's a moot point as they don't even have PC versions.
Really only a couple of utility apps like MIDI mapping apps show their PC or Mac origins and subsequent porting in a comparison of the 2 versions. And even then it's command/GUI stuff like Tutor mentioned not really any performance difference.
If someone is having the Ren/Studio apps or any other modern software run markedly better on one or OS or the other, the user probably needs to make some OS config tweaks that would take less than 5 minutes on either OS, and half that time would be that some settings tweaks take a reboot.
Black Viper, you might want to do those 5 minutes of tweaks, I have a $225 netbook that I was able to triple performance on (which mean 4 tracks and some FX in Ableton

).
http://www.blackviper.com/He added Windows 8 specific tweaks recently, and his charts are dead easy to follow and know exactly what you're disabling to get the CPU cycles and RAM back in your control for the DAW stuff to use better.
Also if you haven't already gutted the bloatware on that PC, use the 30 day fully functional demo of this -
http://www.revouninstaller.com/100% legit, awesome app. Just be careful using advanced mode when deleting registry keys after the apps are taken out. Maybe 1 out of 25 times it misses on the collecting the obsolete keys. And you kind of need to know how to read a list like that to not accidentally delete OS critical entries.
Generally speaking if it shows keys in the Microsoft and /or Windows32 folder that are not labelled with the uninstalled apps vendor name, choose no on the key deletions. But again Revo gets it right most of the time anyways.
To dump you web cache daily and maintain a tight registry CCleaner is awesome and totally free. It's used by network engineers daily and in Cisco schools students are told to use this one.
http://www.filehippo.com/download_ccleanerI've widened the "edge" you mentioned on many underpowered PC's over the years using these 3 methods.