Concerns about accidental button presses are overstated in my opinion, I've never had that problem and my fingers are not exactly nimble and elf like in manner.
Nor will your speed go down, it should increase as you get more used to the switch (true for any switch type really).
My guess is that accidental presses occur more often from muscle stiffness or trying to type faster than one really can than for any reason connected with the keyboard. I get them when the switch's "personality" doesn't agree with mine (most recent example: black alps... all's fine when I'm focused and keen and hit squarely, much less fun when I'm tired and distracted).
I suppose the best way to remove the problem of accidental hits is to get some real touch-typing training to improve one's accuracy instead of dealing with the whole ooops -> backspace routine. For example, I generally almost always know when I hit wrong and then I backspace all the way back and type again, which slows me down and frustrates me. If I could find the time and willingness to spend a full week learning to type better, I'd probably do away with most of it and acquire a +10 wpm bump or so. Some time I will simply have to because I'm paid by the amount of text and not by the hour in my current job (which is translation and in which a constant >400 chars per minute is profitable but >500 would mean roughly +25% income if not more due to being able to take some fast-paced jobs at all, as opposed to merely being able to process more pages in the same time).
For the record, I did notice a tendency to spam more chars per minute with the browns but with reduced accuracy than, say, blacks, that were also quite fast, although relatively taxing at some high speeds (e.g. at the highest speed blacks are probably better than Model M, but Model M is probably better than blacks at realistically sustainable speeds, from my point of view). Then again, switching to a compact layout keyboard from a normal one, you're bound to have accuracy problems.
