I assume when you mean the next size down, you're talking about diameter. I'd recommend a Prometheus 6.03mm or KM 6.04mm tightbore. They'll increase your fps by a little bit, but shouldn't push you over 400fps.
Best minimum range to set your hopup is about 30'. Ideally, you'd set your hopup somewhere where space limitation isn't an issue.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."