PEEK is usually limited to about 250°C (Tg is 143°C, but where it's a semi-crystalline thermoplastic it's ok to go a bit above the Tg for non-aerospace use). I'm not aware of it being used as an additive to thermosets. As Moudoc says, it's also quite viscous; it can be injection moulded with short fibers but I think you'd have trouble infiltrating it into a dry preform (claimed viscosity about 90 pascal-seconds for the 90G grade, cf. 0.02 to 0.05 for typical RTM/VARTM resins). You can buy it as a prepreg with carbon and glass from TenCate.
For the highest temp commercially available plastics suitable for matrixes check out polyimides, such as Cytec's Avimid N, which has a claimed dry Tg of 410°C (375°C wet). However, stay away from ones such as PMR-15 which have MDA in them, as they're bad for you. PBI, a thermoplastic (polybenzimidazole), can go even higher, but I don't know of it being available with continuous fibers. BMIs can be happily processed at 100 psi and maybe 200°C with a free-standing post-cure of maybe 230°C. PIs though may need 200 psi and 320°C. As Moudoc observes, curing BMIs out of autoclave (OoA) is not advisable. ACG do a BMI which they claim can be cured OoA, but they do warn that it will be much more porous.
Re Zylon (aka PBO), we've played with it a bit. It looks useful for armor, but it should be fairly useless for a weight-efficient structure, as laminates of it have very low compression strength (much like Kevlar). Unless it's a tension structure such as a sail, structure-wise you're better off for high temperature with glass (maybe carbon if temperature is <=300°C) in PI, or maybe alumina fiber such as 3M's Nextel, though that may be overkill. If you're tension-only the data from Teijin show reasonable retention of Zylon tensile properties at 400°C. Zylon also degrades quite quickly in sunlight (see North Sails's notes for rigging made of it).