Hi wholper and welcome to the forum.
Let me preface this by saying I am not a big fan of the glycemic index system, and perhaps after looking into it a bit, as I have you will understand why.
First of all, adding fibre to anything will not reduce the carb count of a meal - but I assume that you mean that you are replacing a high carb product such as flour with fibre - in which case the resulting bread will be lower in carbs than it would have been with all flour.
Fibre? Generally speaking the amount of finely ground fibre in a product has a minimal effect on GI if it does have any at all. White bread and wholemeal bread have a similarly high GI. The only way it seems to substantially lower GI is to eat whole grain 'bread' - no, not wholemeal with a few grains scattered through it - but something with a high proportion of whole grains (70 -80%!) Now i have never tried this (in my low fat, high carb, weight gaining days, I used to eat wholemeal and whole grain breads because i thought they were healthy!) - but it sounds more like something I would feed to the birds.
One of the other big problems with GI is that even the tests that Brand Miller and others have done to set up their index - are not repeatable - in other words different test subjects give widely different results. Here is a quick example;
Bürgen ® Mixed Grain bread (Australia)
Bürgen ® Mixed Grain (Tip Top Bakeries, Chatswood, NSW, Australia) 34±4
Bürgen ® Mixed Grain 45±12
Bürgen ® Mixed Grain 69±6
mean of three studies 49±10
- yes that is 3 tests done on the same bread from the same bakery. The GI varied from 30 to 75 and Prof. Brand Miller would have you believe that her mean figure of 49 is somehow magically statistically significant!
Its not an isolated case (porridge/oatmeal has a GI of somewhere between 42 and 80!), and you can check out a list here, to see what I mean;
http://www.mendosa.com/gilists.htm
So different people get different results - as you would expect with differing levels of insulin resistance in the community (yes I did ask how she selected and standardised her test subjects - and strangely it was at about this time the esteemed Professor stopped replying to my emails!)
So, sorry for the long winded response, but the bottom line is that if you are a diabetic - you should conduct your own tests to see what works for YOU. There really is no other way around this - and this is the case if you adopt a more straightforward reduced carb WOL as well - and when its all said and done, GI means nothing without GL, and once you start looking at the glycemic load of what you eat, you are very close to the simple carb counting that Atkins or Bernstein recommend anyway.
HTH
Cheers,
Malcolm