How can I improve this SMPS design?
You do realize that the regulator you've chosen in a boost regulator, meant for creating a higher output voltage from a lower input voltage? And that your input voltage is beyond the regulator's range (1.8V to 15V).
I think it's time to reconsider your regulator, and read the datasheet properly. Personally I never heard of Feeling Technology, but that's not necessarily bad, provided you can get the parts. However, I would go for more common manufacturers, like National or Linear Technology (the latter are more expensive).  

Also, your heatsink is way overkill, IMO. With the given voltages and current you should be able to get over 90% efficiency, so that the complete regulator will consume around 3W. National's Webench has a design suggestion around an LM3150 with an efficiency of 95%.
I'd pick smaller and separate heatsinks for MOSFET and diode, so that the heatsink's shape doesn't compromise the layout. This model  



is only 6.4K/W, so that the temperature won't rise more than ~15°C.  

About the layout: the loop L1/Q1/D1/C7,8 is critical, and should be much shorter than you've placed the parts. I realize that this has to do with Q1 and D1 being mounted on the heatsink, but that's one more reason to see if you can't do without the heatsink altogether. Two reasons for having the loop as short as possible:  


Efficiency. There will be large currents flowing through the loop, and you want to keep resistance losses as low as possible, and  
EMC. Radiation won't be as high as with a regulator working in the MHz range, but even at 300kHz this may cause interference.