The usual mods for the diesel are SDI injectors, de cat, up boost to 19 psi, remap and exhaust. That lot should see you to between 150 to 160 bhp, about 230 ft/ lbs torque and still return 50 mpg. Having had a 180, 3 120's and now the diesel I can safely say you wont be working your way down if you do the above mods. Granted though in standard form you will struggle to keep up with cyclists however with the usual mods you will surprise a few GT TDI drivers
Just to add, the only difference between the 115 and the 101 is the map on the ECU so if you plan to remap it anyway it makes no odds. If you are planning on leaving it standard then the 101 is the map of coice because the 115 apparently have a massive flat spot above 2800 revs.