No, it won't. FSGRW gives me about the same FPS performance I can get with ASXP, the real performance impact SMP can have is related to cloud layers and positioning, drawing lots of broken clouds has a noticable effect but this is the same situation with ASXP or FSGRW - for me it makes no difference performance wise.
This will be reduced drastically with RWC, no matter which weather engine you want to use. In the RWC options make sure to have 'never changes visible weather' checked so you will see much less cloud redraws than without (on mesh tile loading a slight redraw effect still seems to occur, on metar change it's fine).
Regarding the 'killer combo' I have to give some further notes for ASXP users, there are some downsides compared to ASXP:
ASXP draw more noticably detailled wind/turbulence effects
ASXP is smoother in weather injection, currently you have to inject the weather manually with FSGRW after the flight has loaded and then you will have to change settings in the RWC options to really see the current weather (I reported this to the developers and they look into the injection process)
On weather change you will see no cloud redraws but sky colors and haze can suddenly change (I don't remember if this also happens with ASXP or if this is smoothed there).
Aside from that FSGRW gives you better, more detailed looks, more variety in clouds, it also has a more streamlined (and simple) interface and I really enjoy using it, I clearly prefer FSGRW over ASXP but the more detailed turbulences are not that important to me.
No matter what weather engine you want to use I really recommend RWC as this is the key to plausible and great looking cloud formations with SMP. If you want to upgrade your SMP cloud representation this should be the first thing to add.
Here I have some videos of what you can expect from adding RWC to your SMP, weather information comes from FSGRW.