That is most likely not possible. The problem is that FFA doesn't have the most adequate servers right now, increasing player slots would just make it unplayable. The reason why single servers start lagging at 50-100 players is that Minecraft was made to run mostly on a single core, so most modern processors, that have 4,8 or even more cores, don't use close to their full potential, so getting more powerful processors doesn't make much of an upgrade, since they usually just have more cores, and each of those is just slightly stonger than the ones on weaker processors, the reason some anarchy/survival servers have so low player caps when compared to cubecraft is that cubecraft is made up of thousands of smaller servers, one for each match you play in skywars for example, which has only 24 players at most. Right now the only way to get more players on a single FFA server would be getting something like a processor with a very high single core performance, but its power would only be used on FFA, a somewhat small part of cubecraft, so I don't think it's worth the time and money to do it, especially as 40 players is sufficient on most maps
