(OTOH, for an 8-bit variable times a 16bit variable, writing assembler might very well be beneficial, because that's the sort of thing that C is defined NOT to do. It will almost certainly convert the 8bit number to a 16bit number and then do a 16x16 multiply.)
(On the third hand, the AVR multiply instruction is rather inconvenient to use from C, with more that the usual number of restrictions on which registers are used...)