there is nothing that limits the length of a spinnaker pole to J ... boat designers determine the sail plan which benefits their design best which may include the use of large spinnakers to achieve their end result.
in interest of handicapping / parity PHRF ( and other handicap systems ) prefers to use 100% of J as a standard for general spin pole length. There are classes where spin pole length does not conform to 100% of J. Those classes race under PHRF without penality for longer spin poles since their base rating already includes the correction for a longer (shorter) pole. The same applies to non conforming sail plans for those classes. The standard being rated as built conforming to a class. These boats have the option to be rated individually ( outside of their rating /design class )
while an adjustable whisker pole seems to be capable of doing double duty for a headsail and spinnaker... caution needs to be exercised when using a whisker pole as substitute for a true spinnaker pole... in light conditions the loads are of no consequence, however, as the wind and loads build, a whisker pole cannot handle the high loads placed on it by a spinnaker... it becomes a matter of when will it break ( think $$$ lost ) the joint becomes a flex point where loads are concentrated and the weakest part of the pole...while in casual use it may not matter, just overload it one time and watch it break :=( also a telescoping pole carries the weight of 2 tubes adding to the weight that crew needs to deal with... the sail area of a spinnaker is many times that of genoa which goes to the increased loads... a proper spinnaker pole will work under all conditions, only the largest of headsails will suffer from lack of performance, but will not break
the OC400 is obviously for cruising, many handicap rules forbid the use of adjustable poles for both spinnaker and genoas... the race committees don't trust the skippers :=) because it's too easy to gain performance by extending the pole while no one notices :=)