This question gets asked often over on QRZ. Typically there are a number of drawbacks of purchasing a grey market Japanese HF radio:
1. You will not have full band capabilities that you would have with a North American version. The Japanese band plan is restrictive on some bands in terms of band width. This is a software issue which is pre-programmed into the radio so doing a MARS mod will not "open the radio up" to full band transmit.
2. The radios output MIGHT be limited to 50 Watts. Again, there is NO workaround since this is a software issue.
3. The 60 meter band will not be included on the radio.
4. The manual will only be in Japanese.
5. There will be ZERO warranty on the radio should something happen to it 2 weeks after you start using it.
6. There are several other issues regarding Japanese domestic market HF radios. The above mentioned ones are only the ones I recall off the top of my head. Search QRZ for the many, many threads about this.
Some amateurs purchase Japanese market radios from eBay due to the slightly cheaper cost. They are then stuck after they receive the radio and find out of the radios shortcomings. Yes, there IS A REASON the radios are slightly less in cost. Again, the majority of people on QRZ who have purchased one of these radios states that a MARS mod will not solve the bandwidth issue nor will let the radio transmit over 50 Watts.
Buyer beware.
Tom KH0/KC0W