Obviously the step up for the layer 3 traffic from the srx240 to the srx650 is big. So there would be no issues there.
The difference will be with intra vlan traffic between hosts in the same subnet that is currently passing on your switch. Most switches these days are line rate or pretty close to it so lots of host to host traffic is no problem. But a firewall runs a packet inspect process on everthing so the capacity is a lot lower than line rate. You will need to see if the specs on the srx650 will work for your intra vlan traffic.
https://www.juniper.net/us/en/local/pdf/datasheets/1000281-en.pdf