A lot of this is related to SIP overhead, IAX is a totally different protocol. You'll never get SIP to be as thin as IAX, but I can very easily help you reduce your SIP calls to as thin as possible while still giving pretty great voice quality. That's how I run all my hosted VoIP PBX solutions. I'd be happy to jump on a call with you right through one of them so you can hear how crystal clear and low latency they are.