I'll venture to be an idiot for a minute (I am blond, after all)..

So I have a 35i, not a 50i, but..

I would describe it more as a "bump," not a "thump," but I have the same issue.
However.. It seems to be more "the road" than anything else.

Almost like passing over seams on concrete, yes?
And by that I mean the "feeling," as I drive mostly on paved asphalt, so no seams.

I only have run flats now (maybe when they wear out I'll try something else) but I DO feel the road a lot. I'm used to it - I rather PREFER the very SOLID feeling this car gives me. I don't even remember where I read it, but some reviewer said "it has the innate ability of transferring every bump in the road straight to the driver's ass," which is fairly true, but I've come to learn that this is kind of what it's made for.

The Japanese luxury marques all get BAD reviews for NOT transmitting the feeling of the road to your butt. I know that all British car reviewers think everything that normal people can afford is crap, but still..

Yes, I have that feeling in the steering wheel.
As you accelerate (well.. as you go faster) that will probably vanish.
The bad part is that seems to happen at somewhere north of 80MPH.

At oh.. 130MPH, it's like your driving on clouds.
Not that I would know that.