The MAS K 12x8 is perfect and the flex won't hurt you a bit. ST 90 do like RPM and will trounce the OS91 running their bigger prop recommendations. The S2 series will not flex like the K series but will be up a little in RPM. I like the scimitar MAS props now but they don't make a 12x9 or a 13x7. The ST90 is made to run but not on 4 stroke RPM numbers.