I sail a formula135 and have also tried the 175 and 156 (not quite the same as the 155 tho). And I think you can safely say that you won't be losing much in terms of speed to smaller boards at 12 knots.
Going on a beam/broad reach is also not that much of a pain as some make it out to be. In fact, with their widths these boards are able to broad reach much deeper than conventional boards. However, if this is the first time you're moving to formulas it may take a little getting used to. Once you do, I'm sure you won't regret it !
