Gravity doesn't stop working if the power goes off or need repair. Ever had to replace the motor or clear a clog on an sewage ejector? The biggest hassle I can see is that with the new 30', when you slope the pipe properly, will raise the end nearly 8", which may not leave you enough to install the p-trap for the shower. You'd want to check the actual depth of the existing line to confirm before cutting a new trench! Professional concrete cutting companies would make short work of creating a neat, clean, hole in the existing slab. Cracking that much on your own is doable, but LOTS of hard work.