Every PI I have used will out perform a VLF in heavy black sand. It really comes down to performance and cost of a specific PI.
In order of the ones I have owned: the Whites 500 PI was my first I sold it and bought a SurfMaster Pro. I saw and liked the dual tones of the Garrett Infinium - great success with the 8 inch mono coil over an 7 year stretch, then the Garrett ATX hit the market and read the improvements over the Infinium - I had to have one and have not seen anything like it since.
My major PROs of the ATX: Dual Tone, Iron Check, Auto Frequency select, Re-Tune, sensitivity and audio adjustments, compact-ability for traveling, changeable coil size and headphone.
My CONs: shaft cam-locks require extensive cleaning and can be difficult to loosen, weight out of water (I use the Minelab ProSwing 45 when I use the ATX out of the water), the coil is rear mount - not center mount, learning curve, oh yes the location of the Iron check button - it is hard to push and hold with 31" gauntlet gloves on. BUT it is a BIG improvement over the turning of a know that is required on the Infinium. HIGH Cost: it just took longer to pay for itself than the other detectors at a third the price.
Of the PIs I have used the Garrett ATX is the easiest to adjust while detecting - all push buttons, the deepest "seeing", and most sensitive to small gold. Of course, the sensitivity to small gold (nuggets) is what the ATX was designed to be best at. I still own the SurfMaster Pro and the they still have a use in my arsenal. The SurfMaster Pro for its light weight and the Infinium for its 8" mono coil.
All this being said, I use the PIs mainly in the salt waters of New England. I find the Minelab Exclaibur better in the water of Florida. Each detector shines over the other in different locations. Visit a club near you and ask them...they will have more local knowledge of the area you are planing on detecting with a PI and other opinions on detecting.
I use a Whies DFPI and works good in thick black sand. Same location friends with CTX and sovereign GT had no hope. With thick black sand PI is only think to use. DFPI frequency adjust helps. Adjustment will help with falsing and signal depth
With an Excal in heavy black sand, these tips might help you. Work slower than normal, keep the coil very level and at a consistent height off the sand, and try not to bump the sand or set the coil down on it.