Moving a Toliet ?

I'm in the process of moving the location of the toliet to another wall for my bathroom remodel, I am a dedicated do-it-your-selfer. The new distance of the 3" pipe (toliet branch line) will be 7.5 feet total from the existing 2" vent, including one 45 degree coupler. I've asked at the plumbing suppy if a new vent is necessary and they said this layout will work fine. The existing vent is located at the end of the new toliet branch line as it transitions into the main waste line. The venting is downstream from the toliet. Will this be still be sufficient for proper water flow and venting ? Also, when cutting plastic pipe how important is it to make sure your cuts are absolutlely straight, and how does this effect the finished job ? Your advice would be appreciated.

Thanks,
Forgoodmeasure