In a previous article, Dylan Smith said:
Speys - meaning the F-4 had to be modified at great expense. At least
the Speys were a little bit more powerful than the original J-79 engines
so they did actually get something for that money. (Though the RAF would
have been perfectly happy with the standard F-4).

The Speys gave them better range, lower landing speed and better initial
climb, but top speed and high altitude performance suffered.

http://home.att.net/~jbaugher1/f4_22.html

--
Paul Tomblin http://xcski.com/blogs/pt/
God does not play dice with the Universe. -- Albert Einstein.