No drop in pressure whatsoever. But I did install a new oil pressure relief valve kit at the same time. I'm not sure that it mattered, because I had very good pressure prior to doing both.
I have also found that the very small copper gaskets that come with the oil line tend to leak. If you buy a kit from the Roadster Factory, order several of the gaskets that fit between the original plug and the back of the head to use instead. These are a heavy copper seal and will work much better. These are very cheap, so order (4), one for each side of the new line and two for spares, just in case.
One last thing. Be careful with the tightening of the banjo fitting that goes into the back of the head. You don't want to strip the head or break the fitting.