You'll gain the best insight of the SCU DSP's capabilities by reading the SCU manual and the tutorial tech note (SOA-8). Its main strength is performing series of single-cycle multiplications, and if you can't keep the multiplier busy the work is probably better performed on one of the main CPUs (the SCU DSP instruction set is awkward for more general-purpose programming, and the clock frequency is only half of the SH2s).