I just sold my simrad gear but I had a P66 and a Totalscan hooked up to an Evo3 and there was not much between these two transducers down to 100 meters. I think the Totalscan possibly had the edge operating on chirp. I don't think it would be worth getting a P66 if you already have Totalscan. I don't know about the HST- DFSBL but if it's 200/83 there's a fairly good chance it has the same 2D elements as the totalscan.

Left is P66 Right Totalscan at Med Chirp 83

 Thanks mate, how did you find the P66 in the ~50m mark? The HST-DFSBL is also a 200/50 600w transducer. I was told by the chsmith guys that the totalscan transducer is only 250w?

 I found both really good in 50 and not sure if I believe the 83/200 part of the totalscan is 200w. It did as deep as the P66. If they had worked well when my boat was moving I would have kept them. 

Set up correctly they will both work out to 100mtrs no worries. I had the deep water HST-DFSBL on my Evo 2 and could sound fish in that depth at 20knts. Also had the P66 hooked up to my furuno 587 and same results. Deepest I have sounded with both is 300mtrs but that was going slow trolling. It all comes down to the placement on the hull and the type of hull you have.

I have the totalscan now next to a 1kw and do not rate it that high in depths over 50mtrs. Under 50mtrs its a great transducer

 Go to this page transducerguide.simrad-yachting.com/  you can selct what kind of transducer you want, it gives you the basic info, but also had a slider bar you can adjust which will give you the actual beamwidth at any given depth. If that link doesn't work, try this one,  www.chsmith.com.au/Products/Lowrance-Transducers.html and halfway down the page, you will see "comparison slider" in blue, click on that.

I have a HST-DFSBL on my Elite 7 CHIRP, just blows the standard 83/200 out of the water, even at 25 metres, and just keeps getting better. I'm not a deep water bottom basher, but I've clearly marked ground and fish at 100 m, on 200khz, when I've been trolling for billfish.

 Thanks guys very helpful as always

Hey mate, had the HST on a gen 1 HDS unit and was more than happy with the results. Showed good picture at speed and pretty decent detail when trolling speed over marks. Will add some pics, all taken at 4-7 knots. +1 for Ranmars comments about 100m performance.

https://imgur.com/gallery/WYhlH

 But here are some from shallower those depths are in fathoms

This is about 38 metres, at anchor, not calm, swinging out over sand. They are big spangleds, going off.

Same spot, swinging back over the ground with a bit of a wind change.

Slightly broken up, but we were rolling a little.

 You can usually pick them up for $100, s/hand unsused, from people with HDS's who never even put them on. Be aware they are a very narrow beam on 50 hz, really helps with the deeper stuff as you are not spreading your signal far and wide, but will obviously limit the width you are covering at any given time. Did you look at that comparo, I find it really useful for understanding just how much seabed you are actually looking at , at any given depth, on a given frequency with a given xducer.

 Ye I checked out the comparisons, the main differences between the 2 is the cone angle at 50hz, the P66 has a wider angle. But at 200 they were basically exactly the same.

 On my previous boat, and this Lowrance combo works every bit as well on basic sounding, but I miss the bottom lock, and the ability to Shift, rather than go in and do the Custom Range thing