Your rated rpm and governed rpm depends on the power rating of the engine. The isb came with a handful of different ratings, depending on what it was ordered with and what it was installed in. Being that it's a 5.9 ISB, it is definitely electronic, and your rpms are controlled 100% by the ecm.
You might have a top speed of 58 might be because it's gear locked/bound. The only way to tell is by connecting the computer, but to do that it would have cost you an hour of diagnostic, which is 100+ bucks. Even if they did remove a speed limiter in the ecm, with you being at 2300, you're likely not going to gain much in the way of top speed. You'll have to regear the rear axle, or install a transmission with OD to go any faster.
The shop likely said there is nothing they can do, because they don't want to do it, or incur any liability from them doing it. Things can be done to get it to go faster, but realize with your at545, you're already flirting with reliability issues. More speed/power will only hasten it's demise.
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