ATI Stream only goes up to version 2.3. Users of both 2.2 and 2.3 complained about CPU usage bug though. ATI Stream 2.1 is recommended for all 5xxx and other VLIW5 hardware (Up to 6870). AMD APP 2.4 or 2.5 is the lowest compatible SDK for 6930/6950/6970/6990 (VLIW4 hardware), and is slightly slower on lower version cards. AMD APP 2.6/2.7/higher is compatible with 7xxx cards, but takes a big speed hit on 6xxx/5xxx cards.
Also, ATI Stream counts as a different OpenCL platform compared to AMD APP. You can have both installed. This means you can mix VLIW5 hardware with VLIW4 or GCN (7xxx series) and not take a hit on speed as long as you configure your miners correctly, but you can't effectively mix VLIW4 and GCN together without having subpar performance on the older hardware, or being entirely incompatible with the newer hardware because AMD APP 2.4/2.5/2.6/2.7 all share the same install path.