Having done a couple of bargello's, I think you will be disappointed if you use a 2.5" width instead of 2--the pattern will look stretched! Depending on the size you are making, you could increase the "block" (cut for each different size) by 1/2" and the pattern would stay the same--this is one way to make a larger size quilt with the same pattern. I'm wondering about your jelly roll plan though as bargellos work because of color placement being gradiants and usually your strip sets requires one of each of 15-20 fabrics. And even then you may be unsewing some colors depending on curves/twists.
Bargellos are not really hard as long as you follow the grid of the pattern. It does require paying close attention to fabric placement, lots of "unsewing"and resewing (depends on how much the pattern twists or curves), careful pressing of ALL seams and being sure that you "nest" seams in each narrow row.
To keep track of the fabric placement on the grid I suggest you make a chart with each fabric marked (either # or letters ,whichever your pattern uses) and mark off each row as you add it (with a highlighter or cover with a note card,etc). When finished, the huge # of small rows in both direction makes the top act like an accordion and little hard to get borders on right. When I got to that point (on 2nd one--wrestled the first one!) I pinned the bargello top to the mattress and THEN measured it for borders, which allowed me to get correct measurement.
Anyone wanting a bargello that is free pattern and you make in 4 quarters and then combine (like a Trip Around the World quilt) check out Jinny Beyer's website for Navaho Sunrise or Sunset.
Good luck--they are stunning quilts that look much harder than they are--and no points!