If you're using both, the tape would go on first. Which is immediately evident, if you've ever tried to apply them the other way round. There's open debate over whether or not there are any advantages to using both products together, or if each product by itself is adequate.

Pipe dope

Pipe dope; also known as pipe joint compound, is used to seal and lubricate the threads of a threaded pipe joint. It's available for both water and gas lines. It's applied by simply brushing it on the male threads of the pipe joint, before the joint is fit together. Pipe dope never hardens, and will not crumble and fall out of the joint.

Teflon Tape Dope

Teflon Tape dope; also known as polytetrafluoroethylene (PTFE) Tape, or thread seal tape, is also used to seal and lubricate the threads of a threaded pipe joint. It's available in high-density and low-density flavors, for water and gas pipe. PTFE tape is applied by wrapping it in a clockwise direction, around the male threads of the threaded joint. PTFE tape is available in color coded versions (pink for water, yellow for gas), as well as the common white variety.

1. Increasing the pipe size from 25mm to 50mm will definitely reduce the pressure loss along the pipeline, making more pressure available; however, 25mm is already a pretty good size for a residential water pipe and the pressure loss may already be very small. So the difference may not be noticeable. Unfortunately, the problem is likely just the lack of vertical head from the tank to the bathroom.

2. In some places they make tanks with a bladder or spring inside which are pressurized as they are filled to provide more pressure than gravity alone can.