my view is that in rougher water the board poses the main problem. The C145 is fairly wide and any form of chop will besicly undermine it's performance no matter what fin you stick under it. I always try to use the smallest board possible in rough conditions with an apporpriate fin. If however i find myself in a situation where this isn't possible, say i didn't bring the right board or whatnot then i switch to smaller fins. Also, when overpowered i underfin the board, this way i have more controle and more speed though i hold back on early planning (nooooot such a problem when you're overpowered).
My experience with larger boards in rough water it to go in sligtly underpowered with the right fin. See, if you go in overpowered with a smaller fin you'll eighter get a spinnout when you try to get planning or once you are planning and you hit bigger chop (the fin comes out of the water and bang goodbye tail! i had this several times in Leucate and even here in Holland where i sail now and trust me it takes a lot of muscle power to get your board back aligned).
What might help is to use an antiweed fin. My dad had the spin out problem i mentioned a lot in Leucate (he only sails a 220 L freeride board and refuses to step away from it because he lvoes the early planning too much). So we went to one of the shops and they adviced an antiweed and my dad said it did work although his jibes went from 100% to 20 because these fins take a lot of backfoot power to turn.
I'm sure Roger has something to add but as far as this goes i hope it worth something. Though personally, i'd encourage you, if you're going in on rough conditions, to replace the 145 because, especially with your weight, it's too big. I'm at 100 K and i use my 139L only in chilled conditions up to about 18-20 knots if the chop is managable and if the wind isn't too steady (a steady 18 knots mean a deffinate change of board for me, sometimes even at 15).
Haiko, AKA crazychemical