Given that 18 units per em was good enough for Monotype, on the one hand no doubt 1000 units per em is enough, but on the other hand, a number of units that is a multiple of 18 would provide compatibility.
That is not how cognitive scientists now believe that we read.
That may be. But my argument is that even if bouma played a significant role in reading, there could be reasons for difficulty in reading the Arabic script.
Obviously, the individual letters must be important; if one takes a sample of English text, and blurs it so that only the bouma is visible, it becomes unreadable. But it's also true that lower-case reads better than upper-case, despite the latter being more legible, which would seem to indicate bouma is significant.
The question of how much significance is properly allocated to bouma is a complicated and difficult one, and I only have a very limited acquaintance with the facts involved; thus, I felt unprepared to wade into a debate with Hrant over that issue.