I don't think addition and subtraction are too bad in this model. The slides mention that there is a simple circuit computing F(t):=log(1+2^t). So if you have two numbers x and y, represented by their logarithms log(x), log(y), the log of their sum can be computed using F as follows: log(x+y) = log(x) + log(1 + y/x) = log(x) + F(log(y) - log(x)).
Whether this is better or worse than multiplies in the current linear-scale representation depends on how hard it is to compute F, I guess.
Whether this is better or worse than multiplies in the current linear-scale representation depends on how hard it is to compute F, I guess.