The ones I played were EK based and thus could pick SP to avoid any immunities: one very old shuriken (pre-imbue, pre-alc change) and one a week ago (INQ based, HC/survival focused). So, I don't have a direct comparison for the 15 Alc style. The very old shuri char had great dps, but it was absurdly dull to play since all it could do was auto-attack.
I also have a max-DPS thrower (similar to top thrower build from Carpone's U53 list). It does about 3x the DPS of the retooled high-imbue survival INQ, but that's to be expected with a survival build. However, I don't think the INQ could be fixed to be competitive; even with PLs, max gear, Alc for more dice, etc etc. my guess is that it might lift itself up to doing half or maybe a bit more than half the DPS of my real ranged char. INQ just doesn't cut it versus thrower or bow.
If we normalize scaling to RP and assume 350 RP, the factors look about like this:
- x2~2.8 for SP, no ramp-up time & 700'ish is fairly easy on up to 1k if you really work at it
- x2.51~x2.60 for Physical with HE+Sniper, assuming HE+Sniper always crits & 15~16x3x5 profile
- x2.11~x2.2 for Physical with HE, assumes HE always crits & 15~16x3x5 profile
- x2 for RPx2 imbues, ends up ~700+ with full focus stacks
- x1.65~1.75 for Physical (no HE, no Sniper), assumes crits never resisted
- x1.5 for Sneak, assuming you can always land sneak attacks
So, it's very close on whether SP Imbues beat HE+Sniper Physical for scaling and it's clear they don't beat it unless you have a LOT of SP. That doesn't include the recent bug? nerf? to high skull SP imbues and it's a relatively simplistic scaling estimate for HE+Sniper and it doesn't include extra dice on mantle procs.
My worry about the 15 Alc build is that it's not an obvious win. The above list is just the scaling factors. It doesn't include the base damages (the thing being scaled). Physical is likely to have higher base until you manage to get imbue dice up to 35+. So, there might be a win in there for imbue-based builds, but if there is, it's only at cap and it's likely going to be a very small win.
My next (planned) update for my real ranged char only increases it's current values (roughly +18 imbue, +2 sneak, & 15x3 instead of 16x3 physical). This way I know it will be a net gain whereas going all-out imbue has an iffy outcome for a huge effort. If I put that much work into regearing, I want a 100% guaranteed win.