During WWII? Well, for one, the Soviet Union would have been in the dark about many of the internal workings of US-UK relationship. Without Maclean they wouldn't have data about the US's nuclear arsenal capacity, so in order to rival their number they would have to make guesstimates (or use other spies). If you take into account the work by other spies also connected to Maclean's assignment (but not strictly part of the Five), the USSR would probably have been unable or severely delayed in developing its own atomic aresenal to begin with.
IIRC, Philby provided the USSR with details about who British agents were so it's likely that more information about Soviet doings would have been leaked to the UK.
As to how it would affect the US/UK relationship, I'm not entirely sure. There could be a diplomatic crisis of sorts but I'm not sure the extent, giving that they were allies and a world war was going on. Maybe the US would become more private in its dealings and share much less with the UK, or security in their meetings would be enhanced. I also think it's possible that internally, the US would become (even more) paranoid about possible spies in its own system, and earlier.