Did you change to the RA spear in order to switch to a Dyneema wishbone? That would make some sense - the RA spears are designed to for this. For my Omer spear I had to file the groove quite a lot to stream-line it and make it smooth enough for a soft Dyneema wishbone.
The Rob Allen spears have a significantly larger and much, much stronger barb than my slim, stream lined Omer spears.
Personally, I wouldn't change a 6.6mm Rob Allen spear for British fish - it might be overkill but it does the job. Even for their larger diameter spears (usually fitted on models with more powerful rubbers/double-bands), I probably wouldn't bother changing it unless I had some other reason to. Might be handy if you choose to go after conger eel.#
Re. power and accuracy: for accuracy, chances are that the factory set up will usually (but not always) be good, so changing it risks upsetting that, unless you really know what you are doing and have to facilities & drive to check it out. I actually think power (and, to a lesser extent, accuracy) are overrated, for the conditions I usually find myself in other factors are often far more important, e.g. nimbleness, ease & speed of loading, hydrodynamics, reliability, quietness, length relative to visibility, weight (during walk in) & mass (in the water). That's not to say they are unimportant, just that most spearguns will be more that adequately powerful and accurate. [However, my views might be skewed by my preference for my superlight Omer XXV 75 - "the best thing since sliced bread"
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