Thats what you should be doing, cause regardless of whether it's a shit pipe or not, it IS better than a stock pipe power wise and WILL work well with a sport cylinder, and Polini's at that cause it's low revving, punchy, well made, very reliable and relatively cheap, and certainly gonna be a more worthwhile mod to make. If you have any spare change get a Motoforce 17.5mm carb which will make the cylinder perform better and more consistently
To answer your question..yes it would make a difference as it has better porting than a stock cylinder, however as they are just as illegal as a 70 bore, cost the same and are more likely to pack in, you might as well just stick with the 70 kit.