I had the GT-10B and GR-20 both teamed together, sold them for the GR-55.
Much smaller footprint, the GR-55 is highly editable, where the GR-20 sounds are WYSIWYG.
There will always be latecy on any PCM synth (and pitch to midi conversion for external synths), more noticably as the frequency goes lower, due to the physics of note/pitch detection vs string cycles per second (Hz) = latency if it take 2 cycles to detect pitch and velocity.
The GR-55 has a '0' latency COSM modeling engine built in (1/2 of the VB
-99 dual modeling) with some cool analog type synths and various bass and guitar models,
plus a mutli effects unit,
as well as the dual PCM synth engine (2 x GR-20's but fully editable) .
What makes the GR-55 great is the ability to mix the four tone sources (including the normal bass tone) to make a complex composite tone, the PCM synths are great for backing pads and such like.
The GR-55 also has a USB memory stick file player, which I use for playing song intro's and samples, controlled by the foot pedals.
Sounds like your guitarist is only using a very small portion of the GR-55's capabilities.
A couple of examples I use the GR-55 for cover songs are
LynardSkynard - FreeBird - does the church organ type intro and a tap of the CTL pedal changes the synths to violin and orchestral strings section for verse 2.
The Killers - Mr Brightside - does the intro synth + all the backing synths while playing the normal bass lines.
Pink Floyd - In the Flesh - does the wuritzer type keyboard - and Comfortably Numb - the backing synths.
I typically use the Expression pedal to mix the synths with the regular bass tone, heel down for 100% bass and toe down for 100% synth, with it mixed in between.
When I use my 6 string bass, i have a few patches setup with modeled bass on the 4 lower strings, and PCM synth guitar on the two top strings, with the guitar tone routed through the GR-55 amp modeling and distortion effect.