1 - 'ramdisk' doesn't have a meaning within Sybase ASE. So, when you say "Sybase 15.x also provide ramdisk", could you provide the actual Sybase ASE terminology so we know what you're referring to?
2 - Anything within ASE that requires memory (eg, data caches, proc cache, statement cache, in-memory databases (IMDB)) will be taken from the 'max memory' setting.
3 - If the ASE has to go 'outside' to the OS (eg, hard disk, ramdisk) then there will be some overhead for said operation. Generally speaking, staying with ASE memory (eg, data cache) and not having to go to the OS is preferred. Also keep in mind that in many OSs the ramdisk is accessed via synchronous IO ... so while it's usually faster to access memory (compared to disk), accessing memory in a synchronous mode (vs accessing disk in asynchronous mode) may cause its own performance issues. In the end you need to run tests on your system, with your typical workload, to see if a OS/ramdisk will provide any benefit.