I'm looking for a case that will fit under the TV no higher than 150mm but will take high end components sort of like a 'steambox' self build no over clocking required but want to play modern games at 1080p @60 ish fps
Low profile self building is a tricky thing to pull off, if you went with a HTPC case that will give you something more appropriate looking but many are micro ATX with half height card slots, something high performance GPU's tend not to be. You can get full size systems of course.
This Lora PC is an example of the kind of system you might want only 100mm in height, it has two full size expansion card slots but these can only be used with a PCI-e riser which allows you to turn the card on it's side, if you feel like self building they sell the case separately.
Was trying to make a similar build myself, haven't been able to find the right case i.e. the Lora PC is nice, but the power supply can't deliver enough power for a graphics card that is capable of 1080p for modern games.
I'm hoping AMD release an APU that's similar to the ps4 or xbox one or nVidia allow motherboard manufacturers to make use of their maxwell gpu (like alienware steambox).