As Steve, emphasized TESTING IS THE KEY! I found out the hard way.
I, too, had a 300E warm start problem. I researched posts here and concluded (without testing) that I could solve the problem simply by replacing parts. Well, after a new OVP, a new fuel pump relay, new fuel accumlator and new fuel pump check valves ($200 plus and a couple hours!), the problem was still there.
I was convinced it was fuel pressure leakdown but had no way to measure it. I took it to the dealer and they measured 2.9 bar for 60 min after shut off, which is well within the spec. Still, they could not diagnosis the problem.
I then took it to an independant shop (Brooklyn Motoren Werke in Brooklyn, Wisc). They finally solved the problem.
You know what it was -- fuel injection nozzles. Now the car had only 70K miles, but the tech concluded the age and the type of driving by it's previous owner probably contributed to flow and pattern restrictions with the nozzles. (I had run a pile of Techron through it earlier to no avail). They discovered this by examining some of the nozzles and they appeared to have inadequate spray patterns. He then replaced one with a known good nozzle and experienced some improvement in starting. So, nozzle replacement seemed the best bet and it worked. If not, examing the fuel distributor in detail would have been the next step.
So, while you might get lucky from time to time by replacing components by guessing without testing, it will more than likely cost you more time and money than a procedure that includes proper diagnosis.
Hope this helps....