Having installed AGM batteries, I also worried a bit about charging voltages. However, AGMs are so close to flooded cells that most people don''t bother with the 0.1-0.2V adjustment required. <b>But in your case, I would go ahead with the modification to the alternator</b>. There are several reasons:
First of all, a quality external regulator is usually better at charging battery banks. It senses battery voltage instead of output voltage and adjusts the output at the alternator accordingly. Furthermore, it is usually easily adjustable. So if you change battery type, the alternator doesn''t need a trip to the shop.
Secondly, quality external regulators also incorporate battery temperature sensors to prevent thermal runaway. This is a very important safety feature once you get into powerful charging systems. Even Gel batteries can gas and explode if maltreated badly.
Thirdly, a quality external amp will charge faster and/or more gently than a constant voltage charger usually found in internally regulated alternators. This is of particular importance with high-powered systems. Your batteries will last longer with a quality 3-step regulator.
As far as I can tell, you''d be penny-wise and pound foolish not to make the modification. $100 is nothing compared to getting a extra year out of your gel cells, peace of mind, etc.
For more information about battery characteristics, configurations, and charging have a look at my <a href=http://www.vonwentzel.net/Battery/index.html>battery website</a>.