Something worth being said: You can do too much kratom, just like any other opiate. I learned this the hard way when I was trying to wean myself off oxycontins. I approximated given my tolerance and did wayyy too much. To this day, 2 years later, I can't even think about kratom without feeling really queasy D:.
This is a good point that I think some people forget when they're using kratom to wean off of opiates. All the fibrous plant matter and water that you're chugging with it (especially if you're using powdered leaf or extract) can exacerbate the already profound nausea that comes with withdrawal for most people, so if you don't take your doses somewhat slowly, you can end up purging the very thing that's supposed to help you out. On the plus side though all that fiber helps to regulate your bowel movements a bit, which is excellent during withdrawal.
Taking too much kratom can also lead to some jittery and uncomfortable stimulation in some people. This seems to happen more with higher doses, as the stimulatory effects of some of the alkaloids overtake the mu opioid effects on the ol' dose response curve. I'd say this is probably the closest thing to a kratom OD as most people will ever get, but again, since withdrawal is already going to have you jittery from the adrenergic dumping in your peripheral nervous system, the prospect of more jitters is another reason to dose slowly. In lower doses though, the stimulation can be a very positive thing since some people (myself included) find it quite motivating, so you can still accomplish daily tasks and be productive as you get off of opiates.
I'm not too sure what pharmacological action in kratom is responsible for its stimulatory effects, but I suspect that it, at least in part, involves serotonin, since I've had really bad effects after taking several (around 7 to 8) grams of kratom while I was on Paxil. So I'd
be very careful using kratom for withdrawals if you're on SSRIs or SNRIs (or any other type of antidepressant, really!)
So, even if you have a monstrous opiate tolerance, I'd say kratom is still pretty useful, but you may want to give yourself a couple days for your body to acclimatize itself to this unique plant. Really if you're going to go the kratom route, I would stick to moderate doses a few times a day, and even if you are still feeling withdrawal symptoms, ride them out rather than trying to up the kratom, since you'll probably get some adverse reactions trying out heroic doses. After about 2 days you should be used to the lower doses of kratom, and you can titrate your dose down from there or continue on a maintenance dosing schedule if you wish.