Maybe I'm just one of those who prefers the "mine" aspect of Minecraft, but here's what I have to say.
Remember: In vanilla, we generally use the first diamonds that we get on an enchanting table! As fun as macerators, pulverizers, and other ore machines are, good ol' vanilla still has its place in FTB ore processing. In the case of diamonds, vanilla enchanting mechanics beat out all machines: Fortune III will yield on average more than two diamonds per ore. Raid local villages for bookshelves (you only need 2 librarian houses + 1 extra bookshelf). If levels are an issue, many proposals have been suggested already, but here's another one: go caving for an hour or so (it's fun!). Fortune III will pop out on an iron pickaxe a quarter of the time at level 30, and it's infinitely and cheaply repairable. It's your best diamond processing machine ever! Also effective on redpower gems and other ores, so definitely worth the time investment, and faster than finding iron and steel for gregtech's machine casing anyway.
Also: Be sure to look into other options besides a quarry for mining. There are plenty. Buildcraft itself also comes with the filler, which when used on "clear" mode can act as a super-fast quarry. It's just a little messier, and you have to set up a system to collect the drops. There are also the IC2 Miners, which will probably output more ore more quickly than a quarry, and are much cheaper. If you're handy with programming, turtles are also a great way to go. Maybe get a mining laser and an advanced lappack, dig down to Y=10 (or higher if you want more tin/copper) and set to horizontal mode (definitely try this one out sometime). The one benefit to the quarry is that although it is slow,you don't ever have to babysit it.