You listed some great skis. I think that any of them could be a winner for you. Why 155? Are you female? The reason I ask is because I just switched to 165's and I will never go back. They are sooooooo stable and offer speed and edgehold well above the performance of a 155cm SL ski.
As far as the skis go, I have heard that the Rossi this year is VERY stiff, but I could be entirely wrong. If it is the same as last year, it is a noodle and will be perfect for what youre looking for. The Volkl is in the same category, but it will require much more input from the user. They still ski very fast and very smooth though. They will have more rebound than the Rossi (assuming last years Rossi). You don't want an SL:11 WC - it is in the same category as the Nordica SLR - probably at a higher level.
The best retail ski that you have selected, and the best ski for what you're looking for overall is the Elan SLX Fusion (formerly SX Pro). I have skied on my brothers and they are possibly the easiest, smoothest, fastest, easiest, best edgehold, snappiest, easiest (did I mention they are easy?) ski in the bunch. They have edgehold similar to a stock ski, and rebound similar to a stock ski, but everything is tamed down to something more managable for normal humans. They are crazy stiff, but due to the HUGE sidecut, bending them is effortless. Short turns are a no brainer. Rebound is less than a normal race stock ski, but only due to the stuff they put on the top of the ski to settle it down - I suspect that the innards of the ski closely resemble my race stock boards from Elan. They do no thave a speed limit, but I sensed that you can push them to a point where they will stop giving you unlimited rebound... although that point is probably far beyond most skiers capabilities... I didn't find it, but I could feel that it might be there (like I was skiing it on the edge of its capabilities). The SL:11m is also a great ski, but I skied it last year a few times (160cm) and nothing really impressed me. It was okay, but didn't really do anything special. I suspect this is also the case for the slightly softer ST:11. Both the ST and SL are soft skis in my opinion.