I have this exact pump on my 55g which is around 6' above the sump... it works fantastic.
Like was said previously, you'll probably be throttling that pump back in a 20... it's just about perfect on my 55.
You're getting a good deal... I paid $99 for my Mag5 at the local LFS (rip-off artists), but I didn't have much choice at the time since I needed it immediately.
I think 300GPH is about the same flow that comes out of your faucet, so you can see how much water it will pump.
My Mag5 has been going 24/7 since I got it and I haven't even had to clean it yet (going on 5 months).
They're great little pumps.
HTH