You may see occasional short periods when a squadron boss gets promoted from wg cdr to gp capt, and receives his substantive rank slightly before his onward posting (the same kind of thing sometimes happens to station commanders, giving periods of 1* command at stations that normally only have a gp capt). These are anomalies and the current highest rank of established squadron command appointments is wg cdr.
You are right about 617 Sqn having a wg cdr boss from its inception. By 1943 that was the established convention in Bomber Command due to the large size of its squadrons. As noted above, some fighter squadrons retained sqn ldr bosses into the 1960s, but as fast jets became increasingly complex, the number of techies grew to the point that standardisation across the board with wg cdr bosses (and sqn ldr EngOs) became desirable.
Today's misalignment of rank titles with structure is all wrapped up in capability. It would be quite possible to separate one of today's fast-jet squadrons into 2 separate squadrons, each with a sqn ldr boss and a single shift of engineers under a flt lt EngO. That single shift would not be able to sustain 24/7 flying operations - but then neither did fighter squadrons between 1914 and 1950ish, since those sqns were either "day" or "night" fighter squadrons, depending on their aircraft type. Today's fighter squadrons can sustain 24-hour operations as a stand-alone entity, which makes them the equivalent of a "wing" of day- and night-fighter squadrons in WW2 terms.
Similarly, ground-attack squadrons deliver so much more capability than their WW2 equivalents that it would be quite possible to split them into 2; each half would still deliver a punch worthy of the title "squadron".
The reason why we have a small number of wg cdr-led squadrons instead of double the number of sqn ldr-led squadrons is partly administrative efficiency, partly uniformity of command (fewer young firebrand bosses to keep in check!) but primarily about force packaging - you can deploy a wg cdr-led squadron somewhere and it will deliver 24/7 operations due to the capability of modern aircraft and the increased number of engineers available to sustain day and night operations in a larger unit.
Last edited by Easy Street; 23rd Aug 2013 at 21:56.