I'm a Diamond Terran frequently matched up against Masters level players. If a TvP ends before 15 minutes, I'm usually the one winning it, but in lategame I rarely do as well.
I usually focus on Marauder+Ghost+Viking as I've seen top players do so very effectively, but it rarely works for me due to Zealot heavy compositions. Mixing in Marines or Hellions essentially gives me units that die to 2 Colossi shots, so that just doesn't work. My stutter step micro is very very good and I tend to have strong EMPs, but against Zealots with +3 armor (rushing this is the norm) they just don't die before I run out of room to micro, and meanwhile Colossi are ripping through my infantry and my Vikings are getting picked off by Stalkers. If they add Archons, it's even worse.
It feels as if Protoss can just a-move their force and I have to micro Vikings to stay out of range of Stalkers while stutter stepping while landing EMP's, and even if I do so perfectly I still don't come out ahead unless I have a significantly stronger force to begin with or am microing a drop at the same time (which Protoss can defend with instant warp-ins).
The only thing I haven't attempted is mixing in Tanks, but the level of micro necessary to include them in an already hugely micro-intensive force is not even remotely comparable to what a Protoss has to do with their ball. Most don't even include Psi Storm or Sentries and can still power through me despite weaker upgrades and less supply.
I can attach a replay or two later today, if that would help.