Impact of increased pad length in PCB

If you increase the pad, you will probably increase the amount of paste too. Too much paste will increase the possibility of the component tombstoning as the solder melts.

As long as you follow the board design rules that will not cause any problems. You will also slightly enhance the heat dissipation for the 0805 part. You need to watch out for misalignment of the 0805 part. During smd assembly the parts self align using the solder surface tension the larger pads will allow the smaller part to move around more.