I think +3/+2 is fine. (Note that this an updated assessment, I had +3/+3 here first).)
When shapechange is in play--and black ethergaunts can cast it by default--you need to compare to a non-RHD wizard using a sensible PrC. Since black ethergaunts at LA +4 can't actually get any class features pre-epic, be it the slightly OP Incantatrix', the gishy Swiftblade's, or the funky Master Transmogrifist's, I would prefer playing a 20th-level wizard shapechanging into a black ethergaunt than an actual black ethergaunt at ECL 20. On the other hand, black ethergaunts need at least +1 LA, just to balance out the casting, and they do get stuff that's hard to emulate. It's a toss-up between +2 and +3, but I'll go +2, on the basis that +8-16 effective Intelligence* is just about all you get that isn't emulated by shapechange.
The whites have one extra level of casting above their RHD, and can get actual class features to make up for their weaker racial abilities, so an extra point of LA is fair.
(A Master Transmogrifist, incidentally, gets a 720-minute (twelve-hour) duration out of Extended shapechange at CL 18. You can reasonably have all-day coverage for only two spell slots, or even one slot, if using a lot of CL boosts, like Circle Magic. With MT class features and Halruaan Elder class features, a human wizard can give any black ethergaunt a run for their money.)
*A black ethergaunt would have about 18 base + 20 racial + 6 enhancement + 5 inherent = 49 INT at ECL 20. A human would have about 18 base + 5 increases + 5 inherent + 6 enhancement = 34 INT at ECL 20, increasing to 30 base + 5 inherent + 6 enhancement = 41 INT with shapechange into a black ethergaunt. Hence the +20 racial bonus doesn't result in a twenty-point difference at the bottom line; it works out to half that, give or take.