I'm planing on making an intake plenum from carbon fiber and kevlar. It is for a turbocharged car, so it needs to withstand some pressure!

I would suggest aiming for a layup of around 1.5-2mm thick, and a bit beefier on any mounting points and the inlet if it's going to have a hose clamp on it. So my layup would probably be one layer of 200g, three or four layers of 450g, and then another layer of 200g for stability. Extra localised layer of 450 on the mounting points. Solid carbon. 

I'm aiming for 2-2.5mm for total safety to be exact.

Also no, I don't think for this application you really need a uni. Fibre orientation won't be too critical. You need bulk, you won't really have load in one plane more than another. The only place the fibre orientation would really matter is on the inlet if it's going to have clamping force on it, in which case you will want fibres on that plane. Uni may be useful here. 

Made it in pre reg, 1 layer finish, 3 layers of 450... however the shape adds a lot to the rigidity, if i had flat surfaces like yours i would have beefed that up considerably.

The one problem is that I don't want to cover the carbon fiber, for aesthetic reasons. The other thing is that I'm thinking that it won't be necessary though cause the engine is a simple inline 4 motor. That means that it has the intake on one side and the exhaust on the exact opposite. The temperature in the intake side will be under 90-100 degrees(Celcius) for sure. 

I mean I'm not necessarily recommending using a 90 Tg resin for a 350C application, rather just pointing out that your parts aren't going to melt and fall apart. What will happen is that if they are exposed to heat above their capabilities then the resin will soften a touch, which will certainly lead to print through. If the parts are under load and are unsupported, they may distort quite badly depending on their geometry and the degree of load etc. For example on something like a plenum, if you had a hose clamp on the inlet piping, this may distort the mouth of the plenum. Or if you had your intake piping bolted onto a throttle body, and this was unsupported weight that was 'hanging' off the front of the plenum, then this may distort the entire plenum.