Yes, it would be expensive.
And yes, the maximum possible engine speed IS limited by the valvetrain's ability to resist "valve float".
And no, 9000 rpms isn't really very fast when you consider your typical Formula 1 engine can hit 20,000 rpms... and hold that for about two hours at a time over two race weekends.
And YES, a short stroke DOES allow an engine to rev higher. Basically, the longer the stroke, the more force acting on the... oh, wait, wfooshee already explained it.
9000 rpm is nothing on what a typical race engine can hit. And for people who want to go that extra mile and build a high rpm screamer, your typical modifications will include shortening the stroke, increasing the compression (what compression you lose by shortening the stroke, you regain by using higher compression pistons... ) and strengthening the valve-train... which means stronger valves and stronger valve springs.... up to a point... make the valve springs too stiff and the engine will have a hard time moving them.
It's a lot of work. A lot of expensive work. Which is why your typical naturally aspirated modifier costs a hell of a lot more to build than a turbocharged set-up making the same power. For the amount of money you need to make 50% more horsepower than stock on a decent naturally aspirated motor, you could have a turbocharger making 150 - 200% more power than stock... or more.