Rf2 dx11

Any plug ins that write to the screen won't work in DX11 if you previously used them on DX9, any new or recent official content requires the latest build so some cars/tracks won't work.

I'd advise to update to the latest build (Opt-Out of all Betas in the Steam tab) and take advantage of DX11 features and content.
A lot of content now has issues with DX9, because, as mentioned, the DX9 version is several releases behind and no longer supported. So the question is kinda meaningless - yes, you most certainly should switch to DX11, because that's the only way you will be able to enter most online races and use most up-to-date content.

If I remember correctly, the official Le Mans didn't even load properly for me when I tried it with the DX9 version, and quite a few other tracks/cars have various visual issues.

And no, don't use any of the DX11 beta builds, they're old as well. Opt out of all betas and just use the vanilla game, that is the up-to-date DX11 version.