the HU will be under a bit more stress being required to pump out more current than expected... I'd guess it'll be ok unless you have it cranked for long periods. Personally I'd just find a budget amplifier...
A budget quality amp would be your best choice ..rather than stressing your deck .. and actually stressing speakers by under powering and pushing more distortion by trying to drive them..youd get more power and better headroom ...I actually have several nice high quality mini amps that can be hid in the dash at reasonable prices if interested you can PM me ..