Most of well known high speed SIO routines can only work with speeds up to 69kb/s (hsindex = 6). First of all if you want to use higher speeds with SIO2SD (or similar 127kb/s capable device) then you need better routines. Probably the best routines were coded by Matthias Reichl. You can download them together with source code from his site: http://www.horus.com/~hias/atari/#hipatch
In most cases SIO2SD can work with speed up to 127kb/s (hsindex = 0) without major hardware modifications. The only important thing is to add 4k7 ohm resistor (if you do not have one already) between SIO DATAOUT and +5V (pin 1 and 5 of SIO connector).
It should be enough, but more advanced users should remove also at least two capacitors from Atari. Capacitors to be removed connect DATAOUT and DATAIN with ground. Those are small, 100nF capacitors (looks like resistors). In 800XL, 65XE and 130XE computers they have numbers C77 and C78. To show the difference please look below at oscillograms - with and without those capacitors.
As you see it is much better to remove them than to leave them. And even in slower speeds the communication should be more stable.