I would strongly suggest you stick with the factory mounts. Yes, they are pricy but they are specifically designed for your motor and in nearly every situation I have come across, perform better, than the supposedly "better performing" aftermarket mounts that are usually a one size fits all approach.
I have seen and felt certain highly regarded after market mounts turn boats into jack hammers and then the manufacturer refuse a return or refund, so the owner wound up paying for TWO sets of mounts.
I just replaced my Westerbeke mounts last year and it was a very easy decision to go with the factory mounts.