I've got a pair of keystrones with 18sw115-8 drivers, I'm generally running them with a driverack pa+ for filters/limiting and a sae pxm 1450 amp which, when paired with a robust power source, is quite a performer. I've been running very demanding techno music (Enrico Sangiuliano, Joseph Capriati and the likes, low crestfactor and continuous basslines) for hours to an end with appropriate filters, 30-100hz for that box, and a limited setting to 87 volts. I've even run them at about 95 volts long term limit for two hours during peak hour at full throttle. That would be equivalent of about 1150 respectively 1388 watts into that driver(when using the Volt x Volt / impedance minima formula) if I understand it correctly the 700w for the ipal21 Josh mentioned above are based on a much more conservative calculation method so these figures won't necessarily clash, there are many ways to calculate powerhandling, and they never complained or sounded bad, quite the opposite actually, and LOUD. I've run bfm t30 cabs very hard but within recommended limits for a few thousand hours aswell never blown a vc in them either..
The 21ds115-4 or 21ds115-8?
They have the same vc diameter as mine, and powerhandling on paper. The 8 ohm has the same impedance minima as my drivers(6.5 ohm) so I would use these long term voltage limiter settings with them that I use, but I've not been confident running them at 95 volts 99% of the time I set them to 87. The limiter resolution of my drpa with that amp is in 8 volts steps if I recall correctly.
That would be equivalent of approx 60 respectively 65 volts for the same power applied to the 3.1 ohm impedance minima of the 21ds115-4. Take it for what it's worth, I've always had the intention of extracting every bit of performance from my drivers and they never failed me:) but actually I'm a bit surprised I've never blown anything.
Not to say these settings will work for you, I guess limiters can work differently in different LMS for example, just giving my perspective of things.