Key working assumption:  I want and can afford at least a 200hp engine and a constant speed prop....

Van's only recommend at maximum a 200hp engine for the RV-7.
PFA have only approved upto 200hp and have not approved XP-400; IO-390 etc

Lycoming Based

Superior

I already like this company just from the web site.  Obviously the 400SRE....a racing engine would be wonderful....mmmmm.  RV-7s have been fitted with these, and the 400 and 400SRE are the same externally as the 360hp.  Unfortunately the 400 series is not approved in the UK.  The route for getting them approved sounds torturous as well.

Superior XP-360 (180hp)

Superior XP-400 (220hp)

Superior XP-400SRE (250hp)

Mattituck

Looks to be higher performance than superior on first impression for a standard IO-360

Mattituck TMX IO-360 (200hp)

Mattituck TMX IO-390 (210hp)

Turbine Based

Innodyn Turbine

This is not in production yet, but it has been put in an RV-4 and an RV-8 and been flown!  Click the picture and they have videos of it in action...maybe this will be ready by the time I am ready...crikey!

Click here for RV-4 Take off Video, Flight Test and Landing.  They have developed firewall forward kits for the RV-7 !!  It will come in a 205hp and 255hp version.

Second Hand Based

Ex Bulldog 200HP Engine; would be the cheapest solution if I could find one.