In 5 mins you easily climb to 1000ft(which most companies use, at least the ones I worked for, some being (former) major players in the aviation world) before acc. A few even go up to 1500ft which is no harm. 400 ft is a bit low and more importantly, has a big risk of rushing things where nothing should be rushed(memory items, call ATC, in your case, start acc and cleanup - sounds all a bit messy).
So - you are saying that the clever guys in the office of your company made memory items at 500 ft but start acc at 400 ft? Did I get that right? And I always thought our SOP guys in my outfit are "special"(let's be nice today) that make life in busy moments unnecessary complicated....but acc in 400ft....impressive.
For example: We do nothing up to 400 ft except raising gear(with positive rate blablabla), cancel any warnings. 400ft ensure HDG Sel(Or LNAV if suitable), tune radios for missed approach(Single engine procedure), Memory items if required. At MFRA(usually 1000ft except OPT tells us different), bug up, retract, flaps up level change, MCT, climb MSA. Never takes more than 5 mins(and we fly less sophisticated Boeing planes than a 777).