Use it on Mewtwo w/ Modest nature. It will be infinately worth it.
Suicune does not have Roar or Recover. You can use Net Balls on it. It's easier to catch compared to Raikou and Entei for those reasons (Net Ball, no Roar).
The Raikou, Entei and Suicune of FireRed and LeafGreen have rubbish IVs for some RIDICULOUS reason.
Modest Mewtwo is a MONSTER and give it the right moves (Calm Mind, Ice Beam, Recover, Thunderbolt) and you have a REAL threat. I cannot emphasise how amazing it is.
Please, please, please, take my advice above, I guarantee you won't regret it (unlike using it on Suicune).