Hi Blaster - well the smaller boards are usually faster, turn tighter, feel more compact and just offer more "performance". But if this is not what you are looking for, then you can just stick with the 156 - better is always relative! :-)
No, I would say the 156 is really for much heavier riders in theory/or for riders who are on a certain sailing level - usually we hear about long term users of this size being heavier. But again, it really depends on skill set, local conditions, expectations, future goals in windsurfing etc. Glad to hear you are enjoying the board and the sport though.
One thing I have learnt in testing and developing different boards since 1986 (F2 Sunset was one of the first projects I was introduced to testing on) - generally during R&D we start most projects with the existing product and move carefully from there to improve, so smaller changes at first in different directions. So perhaps a board which is only one or two sizes smaller than yours is the best thing to try, to see the differences.
But in the meantime, try the outboard straps and keep the smile while sailing!