Theoretically - no. Depending on which version you have, the advertised maximum cog size was 24T (early) to 26T (late/Pro). However, in practice it is often possible to use slightly larger cogs. This is because manufacturers like to play things on the safe side and because all dropout hangers do not have have the same dimensions.

Presumibly, SunTour would have measued the Superbe's limit using one of their own racing dropouts. Suntour racing dropouts had the smallest dimension from the axle position to the derailleur mounting hole. As this dimension inceases, so does the dimension from the axle to the jockey pulley, which is the dimension that limits maximum cog size. So any other dropouts where this dimension is larger may allow you run a larger cog, if the difference is great enough.

Also, the limit is generally established by positioning the axle in line with the seat stays. Moving the axle farther back in the dropout will increase the distance to the derailleur mount and should increase the maximum cog limit. However, it will also wrap more chain around the cogs and as such will slightly ****** the point of derailleur shifting.

Depending on SunTour's safety factor, the version of derailleur and type of dropout you have, it may be possible to run a 28T. The bottom line is that you would have to try it, to be sure. If you post your derailleur version and dropout brand/model, hopefully someone on the forum will have tried your particular combination and can post a definitive answer.