A heavy B747-400 flight crew departing ORD needed a higher speed than the 250 KTS charted on climbout. They had cleared the speed with Clearance Delivery; but TRACON knew nothing about it and questioned their speed.

Narrative

Departing 28 on runway heading; after 800 FT given a right heading 120 degrees to 040. Aircraft heavy; and we have the high speed/unable ORD 8 restrictions as per SOP from Clearance Delivery. Max performance of aircraft; in addition to thermals is making our V MIN (stall tape) uncomfortably close to our normal departure speed of approximately 280 KTS. I elect to speed intervene whilst we work the SID; and after we level off with still a rough ride at 5000 FT; ATC queries why we're going so fast. I tell him that we're heavy; and he wants us to slow down to 250 KTS. I explain that we have operational restrictions that we need for weight; but I did speed up without telling him by 20 KTS. In anticipation of a continuance to climb; as I'll still need the extra speed buffer until I'm through at least 8;000 FT. I was quite rushed with the turn; and the subsequent level off; plus an intermittent TCAS VFR beneath the TCA. I politely tried explaining the procedure for our hi-speed heavy weight departures out of ORD in the 747; but he seemed to want us to ask for this. Either Clearance Delivery didn't pass along the exemption; or he perhaps was new. Nevertheless; I should have informed him of my slight operational speed need for a few minutes; will be more aggressive in the future.