Wiring diagrams are gonna be very similar. The only problem with wiring will be that (most likely) you won't know what wiring gotcha's exist until you get behind that radio and see for yourself. Changing the tray and connectors will be not difficult. If the MX300 was a quality install to begin with, it has held up well, and no one has monky'd with it since it was installed, it should be pretty straight forward. But, in my experience, those are a lot of "ifs".
If you're gonna swap a radio yourself, make sure you use proper tooling and technique. Finding the source of intermittent problems caused by poor wiring technique and tooling is expensive and exasperating!
Also, you should consider that the KX-155 is already beyond its supported life-span. If you already have one on hand and are looking for an easy swap, so be it. But, I wouldn't go out and buy one. There are ready replacements out there that won't cost much more and will be a much better investment in the long run.