Here are answers to your questions:
* Should I expect faster tracking than Roland's solution or is it going to be kind of the same latency?
MIDI Tracking will generally be faster. But if using internal sounds the VB99 may be faster because it doesn't have to do such accurate pitch-detection compared to when it's outputting MIDI.
However, there will always be some latency, particularly on the very lowest notes. Using pitch-detection it's not possible to get very fast tracking on very low notes because the frequency is too low to measure quickly.
* Is there any difference in terms of tracking and latency time between the G2m, B2M and i2M musicport boxes?
Mostly, there isn't any significant difference. However, what does make a difference in the "feel" of the tracking is the note range each unit is configured for. For example, on guitar the G2M feels faster than the B2M because the B2M is configured for the lowest bass notes, which the G2M can't detect. To address this, the i2M can be configured in 3 modes: guitar, 4-string bass and 5-string bass. The guitar mode is identical to the G2M, and the 5-string bass is identical to the B2M. The intermediate 4-string bass mode gives a balance between the two (there is also a "wind/voice" mode which is similar to the guitar mode for tracking speed).
The note-on latency (going from nothing being played then plucking a clean new note) is the same regardless of the mode set. (Of course, the latency depends on the frequency of the note played.)
* Is there any noticeable latency difference between the USB unit vs. the MIDI units through a MIDI to USB box?
Not usually, but it depends on your setup. Into a computer, the USB unit can be slightly faster because there isn't an extra MIDI->USB conversion stage required. With MIDI hardware (keyboards, etc), the G2M can sometimes feel faster than the i2M on a computer because the hardware sound generation often has lower latency than a computer.
A particular advantage of the USB unit is that it has extensive configuration options with the MIDI units don't have.
* Does the i2M work with Linux? I understand it's not supported, but, has i been reported by users to work?
Yes, it should work. It uses standard USB-Audio protocols so Linux should support this, and we have customers using it on Linux. However, the desktop editor doesn't work in Linux (only Windows and OSX are supported). The i2M's settings are stored onboard, so you can use an editor on a PC/Mac to configure it then use those presets on Linux.
* Do you guys ship worldwide from the eBay store or is it UK only?
We can ship worldwide. However, depending on where it has to be shipped it can work our cheaper to get it from one of our distributors if there is one in your country.