No, relocating the boiler won't make a difference to your water pressure. The pressure at the shower is the sum of the pressure available at the point where water service enters your house, minus the pressure losses due to pipe friction (related to pipe diameter and length), loss in the boiler, and loss due to the rise in elevation (0.4 PSI per foot, or 9.8 kPa per meter). The sequence of the components and their relative placement in the house don't matter.
Re-plumbing may make a difference if the pipe path is very long or under-sized, but for a hot water use frequently we prefer smaller-diameter pipe so that less time and water are wasted waiting for the hot water to arrive at the fixture.
You may also have flow restrictions. It's possible the boiler could limit flow to assure that its discharge is at the required temperature. It's also possible that your shower valve or shower head have intentional restrictions to reduce water consumption, or simply debris. Comparing the flow at another shower, bathtub filler, or sink may help you identify what's going on. You can literally compare the flows by measuring and comparing the time required to fill a bowl or bucket at each of the fixtures. You also can compare the flow of hot and cold at a fixture to identify whether the boiler is creating a restriction to flow.