Admittedly, I've never had an Eagle in A rated gear... but my Eagle never broke 400 at full boost... My Courier cruises at 295 and boosts to slightly over 400.
You must have an absurdly lightweight Courier. I'm using military composite plating for mine, but aside from that, I'm pretty much fully A-kitted with a cookie-cutter dual pulse / multicannon loadout. I'm weighing in at around 104 tons, and I can barely break 390 at boost and 288 while cruising. I wouldn't think that a paltry 8 tons from my bulkheads would decrease my speed that much.