For instance the Filco keyboard I am looking at http://www.diatec.co.jp/en/det.php?prod_c=805 claims to use a through hole key switch and diodes for every key switch to prevent "ghosting" which I feel would be somewhat important for the gaming and typeing I do.
Lots of keyboards have no ghosting. For instance, the Rosewill keyboards, which are frequently on sale for $55.00 or less have NKRO and no ghosting. Any keyboard with NKRO will have no ghosting.
It really does suck to get a keyboard with ghosting. I had one once before many years ago it was highly annoying.
Just make sure whatever keyboard you get that it says NKRO and you will be safe.
You've confused two different concepts. NKRO and no ghosting are only slightly related.
NKRO means the keyboard can send more than 6 keys at once to the host. The USB HID spec defines 7 bytes for sending active keys to the host: 6 non-modifier keys, plus one byte of bit-flags for the modifiers.
"NKRO" is typically implemented in a couple of ways. One is to support PS/2 mode, which places no constraints on the number of active keys when the keyboard is physically connected via PS/2 (under USB, the 6 keys + modifiers limit still remains). The other way is to implement a USB hub within the keyboard and to break the keys up into multiple USB keyboards (e.g. Noppoo Choc Mini).
Anti-ghosting refers to the measures used to remove (or mitigate) ghost keystrokes. Ghosts occur when 3 active keys make up three points of a rectangle on the physical matrix. This causes the fourth "corner" to go hot. This is why keyboards with no anti-ghosting measures are called "2KRO": although not every combination of 3 or more keys will cause ghosts, you can only be guaranteed to avoid ghosts if 2 or less keys are active.
Anti-ghosting is done in a few ways. On keyboards with a PCB (i.e. not membrane) the best and most reliable way is to fit diodes between the switches, to prevent the back-flow of current. This is a 100% complete solution. Partial solutions include implementing a sparse matrix (i.e. empty points in the logical matrix to help avoid making rectangles), arranging the matrix strategically such that keys that are commonly held down (especially the modifiers) don't share the same rows and columns, and firmware logic to remove the fourth key given that three keys known to be prone to ghosting are active (obviously this is a hit-and-miss approach).
So your rule has a couple of exceptions. A keyboard may be 100% guaranteed to have no ghosting because of diodes, but this doesn't necessarily mean that it must support PS/2 and in that case you only get 6KRO. Conversely, a keyboard may support PS/2 and so be advertised as "NKRO" but may only have partial solutions to ghosting -- not the full diode-based solution.
