Perhaps the SY99 does provide different keyboard velocity curves [1]? I
own the first version of the DX7 and it's keyboard comes with the issue
you described, but for all external gear and for the DX7 itself too. I
like it, since it enables to control the averaged wanted dynamic good
for my needs. I avoided to use update or customized DX7 software, but I
guess velocity curves can be add to all Yamaha synth since the 80s, so
assumed the SY99 shouldn't provide it by default, there for sure are
EPROMS or discs etc. available. IIRC even when I hit a key as fast/heavy
as possible, the velocity doesn't reach 127, but 116 or something like
that only. I suspect that you are accustomed to your SY99 sounds and
your drum machine, but that the default setting is optimised to averaged
dynamic ranges that fit to the kind of keyboard mechanic velocity
resolution and MIDI velocity resolution. It's very likely that there is
an app, that allows to transform incoming velocity data. This perhaps
isn't as good as a keyboard that provides different velocity curves, but
might satisfy your needs. Some virtual synth perhaps do provide velocity
curves too.
[1] "See page 253 of your SY99 owner's manual, Utility Mode/2. Velocity
Set on how to set up the velocity curves, this may help some. I have not
checked my SY99 for velocity output, but for years my SY77 routinely
transmits around 115 to 118 maximum velocity and it has not been a
problem using it as a master keyboard/controller for the last 16 years
with a wide variety of synths./modules. Hope this helps." -
http://www.yamahaforums.co.uk/forum/viewtopic.php?f=55&t=1285