Your rpm seems very low. That West engine is supposed to be one of the strongest 50 size engines out there. For comparising purposes my MVVS .49 swing the same prop at 13600 rpm and my ASP XLS .52 a couple of hundred more. Are you sure that you have leaned it out enough? Well broken in?? What pipe are you running?? What fuel??

I think the 13-4W will be more sluggish and not make you very happy. That engine needs to rev to make its best power. I would get the 12-4 and try to squeese the most rpm out of it. If you are not using a very restrictive pipe or bad/wrong fuel that engine should spin that prop at 14K or better and pull like an ox