Secus are normally wrecked because most regular ''skilled'' people rarely play as one...
pRiMoz got the right point. There's a tendency that president hardly survives when he's not regular, because most of the regulars play as terro then (in the best case they split between cops and terrors, and leave president and his newbie protection team totally alone), and the president from regulars survives in most cases because almost every regular switches to security.
You can give minigun to newbie securities, that won't change anything against well-skilled terrorists team.
In the mean time, MP5 driveby is an inevitable part of security gameplay, it will be even harder without. As for running weapons, they've got sawnoffs and that's enough.
Certainly the game must be balanced because in 90% cases it's one-sided game. But unfortunately your suggestion doesn't seem to be something that can help. So,
-1.