Hi Labra,
You have more or less answered your question when you said...
"Apparently the device needs more RAM, not just a larger storage, to keep certain files that will be used in real time: no time to call them up from a HDD". The extra RAM that you refer to is the expansion memory board which is used to hold the wave samples that are used for expansion voices.
If you don't plan on using any expansion voices (Yamaha Premium Packs, third party custom voices, or your own custom voices) then you don't need the expansion memory.
The wave samples must be in fast memory (the built in waves are in ROM for example), and in order to avoid having to reload the expansion voice samples into volatile RAM each time the power is turned on, the Tyros5 and 4 use nonvolatile flash RAM which is much more expensive than normal dynamic RAM, although Yamaha are still having a laugh charging what they do for their expansion boards!
Although the hard disk in a Tyros4 (and I assume a T5 too) is a standard 2.5 inch laptop SATA drive, it is actually connected to the system internally via USB, using a SATA to USB interface card fixed to the disk drive, and this limits the transfer speed. Even if you replaced the standard mechanical disk with a SSD, the transfer speed would still be limited by the USB interface.
A computer OS such as Windows can use a 'page file' on the system hard disk if it runs out of physical RAM for an application. This will slow things down, but it shouldn't really matter for such things as word processing and the like, but it could cause problems if you are using virtual instruments for example, which really need to be fully loaded into real physical RAM. In many circumstances adding extra RAM to reduce the use of a page file is one of the most cost effective things that you can do to improve your computer's performance.
The Tyros OS has no provision for using a page file on the hard drive. In fact Yamaha keyboards are self contained units in terms of the OS which is completely held internally in non volatile memory, and is not loaded from the hard disk (most of them don't even have a hard disk). The hard disk on a Tyros is mostly just used as a store for non preset files such as styles, registrations, voice edits etc., and the keyboard can run perfectly well without it apart from the Audio Recording function (which can only be done to the hard drive), as I discovered recently when the interface card on my T4 hard disk failed! I have now fixed the problem, but I used the keyboard for a couple of weeks without a hard disk, using a USB stick to hold files that were previously accessed from the hard drive.
So the 'wisdom' is, if you want to use expansion (Custom) voices, you need to buy and fit an expansion memory board.
Regards
Ian