http://www.amazon.de/Focke-Wulf-187-Der-vergessene-Hochleistungsj%C3%A4ger/dp/3925505660/ref=sr_1_1?ie=UTF8&qid=1339344347&sr=8-1
http://forum-marinearchiv.de/smf/index.php/topic,4926.0.html
This German forum post is quite extensive and is legible with google translate. It draws heavily from the book I linked above to demonstrate the technical source that they draw from.
With twin DB601 engines the single seat version of the FW187 could achieve over 400mph fully loaded in level flight. This is from the German book cited above. It also covers the ability to climb and dive, as well as its maneuverability. I don't own the book, otherwise I would flip it open and quote more technical details. Feel free to browse the forum posts in the link above, as it has much of this information.
Note that these weren't projected performance figures, but actual achieved figures from FW and Rechlin testing records.
The P38 finally achieved same speed in its L series variant, which entered service in June 1944, more than 4 years after the FW187 could have been operational and using engines at least 500hp more each, so more than 1000hp more together than the twin DB601 engine FW187.
The P38 was more versatile, true, which made it similar to the BF110 in its role, but the FW187 was designed simply as a long range interceptor/escort fighter and excelled at that role due to its high speed relative to the likely opponents (the Spitfire Mk. I having at best 350mph, which is 50mph slower than the FW187) and the superior tactical doctrine/team tactics developed by the Luftwaffe prior the WW2 based on their experience in Spain.
As to those 'who know more than me about air combat', Udet and Goering were the selectors of the Bf110 over the FW187 and had no technical training or modern combat experience after 1918. Udet was a test pilot that never flew the FW187 and he notoriously selected the wrong aircraft and crippled Luftwaffe production by his very poor decisions in just about every facet in the technical branch.
Wolfram von Richthofen, one of the 5% of the Luftwaffe officers that had an engineering degree discovered the Fw187 project and pushed it in his capacity as the head of the development branch, but lost his job after Udet took office as the head of the technical department over Wimmer, perhaps the best technical mind in the Luftwaffe.