Sorry guys, but:—
1) Until now, I’ve never heard of any limitation on sideslipping a 150/2 or 172 with full flap. Cessna seem to be equally puzzled by the various theories floating around on this and answered a query from a pilot on sideslipping—their response is given on the ‘Landing Long’ thread in the Private Flying Forum);
2) The only sideslip limitation I’m aware of on the PA28 (Arrow, anyway) is that “prolonged slips or skids which result in excess of 2000 feet of altitude loss...which could cause uncovering of the fuel outlet must be avoided as fuel flow interruption may occur when tank being used is not full”.
Hope this helps.
HP