Turning the OD of a gear can have some negative issues as people have stated you are removing 10% of the active tooth depth. Depending on the tooth counts used this can have a much more dramatic effect than a 10% reduction in torque capacity. One of the factors that drive the rating of a spur gear is known as the contact ratio. It is essentially a measure of what percentage of time one tooth is in contact verses two teeth in contact. When you truncate the OD of one gear you shift that contact ratio closer towards one tooth in contact more often than two teeth. You could imagine when the load is shared between multiple teeth more more load can be transmitted.
Additionally many times gears have what is known as tip relief. Where the involute is modified slightly on the tips of gears to allow them to enter and exit mesh more smoothly especially when a load is applied. I do not know if FRC gears have tip relief machined in, but if they do you will remove it and possibly cause the gears wear more quickly and be noisy.
How many teeth are on the CIM pinion? Would it be possible to use a larger pinion? If you are trying to keep a specific ratio vexpro might make just what you need. They appear to have some CIM pinions with modified addendums or shifted centers. (The modified addendums will allow you to add .025" of center distance without the increase in backlash or shaving the OD of your gears basically lets you set the center distance up as if the gear arrangement had an extra tooth).
See page 9.
http://content.vexrobotics.com/vexpr...ence_Guide.pdf