John Piper is a seven point Calvinist. I hold to all seven points as well. John Frame is a six point Calvinist who disagrees with "the best of all possible worlds" view [John M. Frame, The Doctrine of God, (Phillipsburg, New Jersey: P&R Publishing, 2002), 170-1]. In looking for a source arguing for the 7th point I found this bit by Jonathan Edwards and quote it below as a resource for others looking for sound theological reason on this point.
The following is from Jonathan Edwards, "Concerning the Divine Decrees in General and Election in Particular" in The Works of Jonathan Edwards, vol. 2 (Carlisle, Penn.: Banner of Truth Trust, 1974), 542, [underlining added].
§ 59. The objection to the divine decrees will be, that according to this doctrine, God may do evil, that good may come of it.
Ans. I do not argue that God may commit evil, that good may come of it; but that he may will that evil should come to pass, and permit that it may come to pass, that good may come of it. It is in itself absolutely evil, for any being to commit evil that good may come of it; but it would be no evil, but good, even in a creature, to will that evil should come to pass, if he had wisdom sufficient to see certainly that good would come of it, or that more good would come to pass in that way than in any other. And the only reason why it would not be lawful for a creature to permit evil to come to pass, and that it would not be wise, or good and virtuous, in him so to do, is, that he has not perfect wisdom and sufficiency, so as to render it fit that such an affair should be trusted with him. In so doing he goes beyond his line; he goes out of his province; he meddles with things too high for him. It is every one’s duty to do things fit for him in his sphere, and commensurate to his power. God never intrusted this providence in the hands of creatures of finite understandings, nor is it proper that he should.
If a prince were of perfect and all-comprehensive wisdom and foresight, and he should see that an act of treason would be for the great advancement of the welfare of his kingdom, it might be wise and virtuous in him to will that such act of treason should come to pass; yea, it would be foolish and wrong if he did not; and, it would be prudent and wise in him not to restrain the traitor, but to let him alone to go in the way he chose. And yet he might hate the treason at the same time, and he might properly also give forth laws at the same time, forbidding it upon pain of death, and might hold these laws in force against this traitor.
The Arminians themselves allow that God permits sin, and that if he permits it, it will come to pass. So that the only difficulty about the act of the will that is in it, is that God should will evil to be, that good may come of it. But it is demonstrably true, that if God sees that good will come of it, and more good than otherwise, so that when the whole series of events is viewed by God, and all things balanced, the sum total of good with the evil is more than without it, all being subtracted that needs be subtracted, and added that is to be added; if the sum total of good thus considered, be greatest, greater than the sum in any other case, then it will follow that God, if he be a wise and holy being, must will it.
For if this sum total that has evil in it, when what the evil subtracts is subtracted, has yet the greatest good in it, then it is the best sum total, better than the other sum total that has no evil in it. But if, all things considered, it be really the best, how can it be otherwise than that it should be chosen by an infinitely wise and good being, whose holiness and goodness consists in always choosing what is best? Which does it argue most, wisdom or folly, a good disposition or an evil one, when two things are set before a being, the one better and the other worse, to choose the worse, and refuse the better?