My 4510 came with the 'dual' USB supply, but it works perfectly OK when plugged in to a single USB port. I only have a EDGE signal locally and haven't noticed any heat issues using the modem on the dual cable or plugged in to a single USB port, this may be a lot different with a 3,5G signal and the power requirements of the modem.
3G modems have a maximum power output of about 2 Watt and a maximum power consumption of nearly 3 watt. So you could have about 1 watt of heat to get rid of without much of a heat sink. The power output is controlled by the bit error rate so if you live next door to a tower your modem would only be transmitting milliwatts and there would be no heat…so maybe if you connected it to an external antenna it would not need to transmit at full power to get the job done? Less heat and better speed!