"...Life would be easier if we had TCP/P Autotune like.."

what i need is a firewall that support connections of 30, 50 and 100 Megabit/sec, is this possible?

Yes, assuming the backhaul supports 100Mbps.

If you insist upon running the firewall on the same server as everything else, expect some contention. That's a fact of life. The busier the system is processing other data, the longer the delay will be to inspect packets, etc. Likewise, the busier the firewall daemon is, the less processor time will be available for other tasks.