As such you really need as high a DPS as you can manage for them, but a spinal mount will likely have less DPS for two reasons. First would be that being a spinal mount it fires far slowly than standard, and then take into account the chance of missing with those fewer shots per minute.
DPS on spinal is exactly identical to non-spinal.(short of some rounding maybe) What increases is the statistical variable of Variance, so your damage output is more susceptible to luck and chaos as a shot of many separated units would do.
Keep in mind however that you get penetration value in trade, so I would net them to be slightly better in matters of armament.
The main counter for spinal parasites is though that you lose PD capacity. Firstly, beam fighters are more for PD, finisher, and civil hunt roles anyway. In each of these, strong penetrating alpha damage does nothing, and rate of fire is either necessary or just more comfortable.
When you really want a beam attacking fighter however, you'd still want it to be a PD one, because when you approach an enemy, often enough missiles will come your way, and you are simply better at shooting them down with fast firing. The spinal only adds penetration, not damage performance, so I think it is not really worth gaining this little advantage, and vastly sacrificing the useful role as potential PD-aid in trade.
...Not to mention the stealth is much worse too.