I have thought about getting the Legacy (the wide version of the SL), but the only thing holding me back is that it's a directional twin...the only reason that I would like a true twin is because I don't want to have to change my stance or form when switching from regular to goofy.
I would say that I'm definitely more of a carver than a park guy, but I still like to hit the park a couple times and bust out a grab or two. My instinct is telling me to go with the 158, but I just want to be sure that it can still handle being put through a couple of park laps each time it goes out.
The changes you would need to make to ride switch on a board like the Legacy or even the Cobra X are minimal - if you can ride switch, you can ride this board switch the entire run if you wanted to.
In my opinion, the benefits of a true twin for riding are way overrated. If you are riding switch 50% of the time and are in the park 100% of the time, it does help a little... but if you are riding switch like 5% of the time and outside of the park 80% of the time, I don't see why you should make this type of trade off.
It's like buying a high riding SUV "in case you want to go offroad", when in reality 99% of your driving is done at like 65 mph on paved road (where a lower, stiffer suspension would give you a much better corning performance).
Again, remember that I said since you have ridden and like the 155, you should get that. That being said I have no doubt that a 158 is perfectly fine for a few laps in the park. The Proto is still an incredibly park friendly board... and adding 2% to the overall length isn't suddenly going to make it a hassle to ride. It will bit a little bit heavier (but my rough estimate about 2 ounces heavier) and a little bit stiffer. You might not "prefer" the feeling... but 3 cm is a subtle difference.