The BI rads all have shrouds built in, but they have almost no plenum so the area under the fan motor is obstructed. This is more of an issue on the "push" side than on the "pull" side.
There is a "coolshroud" designed to add plenum. I've just used old 120mm fans, with the mechanism removed, to add plenum, but there's the possibility that this might reduce flow (as compared to the coolshroud) due to venturi (fan bodies have a "throat"). I would guess that this effect is minimal, particularly as the air coming off the fan is not a column but a torus - at least not that close to the fan.
You could also just use longer screws, with possibly nuts to hold the fans away from the BI built in shroud and just wrap with some kind of tape. You can get aluminum tape at the hw store that might work best, but if it's a temporary test you could use anything, bearing in mind that duct tape adhesive is hard to remove
All that said, your coolant temps are going to be relative to the temp of the air going through the radiator (well,
duh), so if your inside-case air temps are a few degrees higher than room temp, then that's your "baseline". Not anything earth-shattering, just something to keep in mind when evaluating temps (and laying out your next case
) - and you might try reversing fan direction (even though it's against thermosyphon) just to see what that does... Oh, and those few degrees might what's standing between you and that overclock you want - or maybe not.