Ahh .. Fung Shui, the more you know, the more  you get. 
If you want to do a concrete tiles table top with more than 600mm tile you have to accept joints unless you willing to pay big bucks Get those 800x800 tiles and cut to size .
Hot wall is not an issue if the wall can withstand the heat, anyway, do not think you are doing "Tai Chow" at home all the time.
As for silicone for the gaps, here is something for you to think about - silicone is soft and with the humid and hot condition in the kitchen can cause the silicone to get fungus. 
DIY ah .  . . wahhh... share your result lah. 

After 6 months of renovating my house, now planning for concrete top for wet kitchen.
1- The slope was in my planning but again the Feng Shui was saying stove have to be lower than the sink.    . So.. If I did the slope, then the stove is higher than the sink.  
2- Yes, you can't find 620 mm tiles. But if I do the slope, it will be 600 tiles + 50 slope = 650mm.
Side wall 600mm and base 575mm. (my planning)
3- I find 600mm for stove is bit small. Having 600mm top,  the wok edge to the wall left between 25-50mm. The wall will be very hot.
4- Wall eveness is not a big issue here, cos u will put silicon to the join. And it so easy to clean even if it is wet.
As for saving may be not much.

I'm planning to order the panels and install DIY.   

I don't know why so many people have issue with FS.  I follow,  no issue with FS and already integrate to my just design. Is not confuse at all.