Hi Mikey,
Since you mentioned that you are able to get a maximum of 6 Mbps of speed across the vpn , there are lot of parameters that needs top be considered for giving answer to this question.
# Do you have any other traffic running on the firewall along with VPN traffic.
# As per the data sheet i see 85 Mbps of data supported (ignoring packet size now) across vpn , however you have mentioned 30 Mbps , do you have any other features like IPS, Appid, UTM EWF, Antivirus, Antispam etc
# Do you see data plane CPU spikes ( when the VPN throughput is 6Mbps).
# Any policer applies on either end to rate limit the traffic?
Regards
Hemant