One of the things a subscription provides is current lists and definitions (AV/spam/ads/malware/bad-ips). With alternative firewalls that are not sub-based you rely on public lists (generally they’re very good) that may not be as up to date. So depending on how the sonicwall was configured / being used, they may notice a difference, or may not.
Now it looks like your question - a solution that is faster than 250mbps after IPS / filtering - is a hardware one. You can install RouterOS / pfSense / OPNsense / untangle on any hardware and still purchase support from them. All of the solutions can achieve greater than 250mbps with the right hardware.
If they are a growing company with a 250mbps connection to the internet, I would plan for gigabit (5 year planning). Some services may be later off-loaded to a dedicated appliance (IPS) to reduce the load and extend the longevity of the firewall however. Something to consider anyway.
After all that, it really boils down to budget.
https://www.amazon.com/d/Computer-Servers/Supermicro-SYS-E300-8D-Intel-D-1518-Server/B01M0VTV3E/ref=sr_1_3?ie=UTF8&qid=1548353283&sr=8-3&keywords=supermicro+server
As one solution.