There has already been a few performance fixes in sponge - the major one being entity-activation-range. You can configure it in config/sponge/global.conf.
It is very tricky legally to package both Sponge and Minecraft into one package and distributed by us. Both Forge and SpongeVanilla instead modify the server jar at runtime, resulting in the same performance but with none of the legal grey area. It’s one extra step to download the minecraft server separately.
This is exactly what Sponge is designed to do, just like Bukkit. Be backwards compatible wherever possible. Potentially means that plugins that were designed for 1.8 could still work perfectly with Minecraft 2.5, or whatever they go with.
Yes. We often have to work in compatibility with Forge to ensure that mods get integrated with Sponge. Using a version that’s older or newer than what it was built with could lead to crashes, or even worse, silent failures.
Right now, SpongeVanilla can only be run as a server. This may change in the future, but right now we’re focusing on Serverside stuff.
SpongeForge can run on the client just like any other Forge mod, however it will only do anything when you’re playing in Single player (effectively a local Sponge server), not when you’re on another server.
This has been discussed at length in a few other topics, so I won’t repeat all of that again. OMG, you're getting rid of /plugins?