I read IMP003 datasheet and FCC testing reports and found that IMP003 802.11n was tested with MCS0 (6.5Mbps 20MHz) and MCS7 modulation (65Mbps 20MHz). If I set my Wi-Fi accesspoint point to 20/40MHz, will IMP003 use 40MHz?
No, the IMP003 only supports up to MCS7 which is 20MHz. This is totally backwards compatible, though - it just means that the maximum speed modulation only requires 20MHz bandwidth.
If your AP is set to 20/40 then devices that support modulations in excess of 65Mbps will use 40MHz bandwidth, but as they move away from the router and their SNR decreases they will fallback down to 20MHz bandwidth usage.