...mumble mumble thinking outside the box something something awesome FAS flight profile mumble mumble...
I like to outfit every ship as shield tank just to see if it's viable, and more often than not with the new guardian shield reinforcements it really works.
What it really comes down to (the everlasting debate between shied/hybrid) is how long the engagement lasts, combined with the shield multiplier and recharge rate. You kind of have to do a guesstimate in your head (cos we don't have the true math, as far as I know), so to whether you can get enough HRPs on board to justify a biweave. If you can't (FDL/Mamba/Asp Scout, others), then a shield build is the way to go and cells instead of HRPs. Most important though, in determining effective health, is engagement length.
I reckon my average fight lasts about 10 minutes with an equally skilled player in a similar ship, so I have to ask myself, how long do my shields take to go down. Let's assume its a good opponent and chaff is pointless cos all fixed (chaff vs gimbals increases time to live better than any other module), so my shields will be down after about 2 minutes. 45 seconds later (1m to be exact due to 15 sec cooldown), my shields are back up at half strength, so they then last another minute before having to recharge again. Over a 10m engagement, a ship with a 500mj biweave (FAS is more like 420, but meh) gains the following effective health... 500mj for first 2mins. After 3 mins, 750mj (7 mins left), after 2 more, 1000mj, after 2 more, 1250mj, after two more 1500mj, and as long as I don't let him get too much damage into my hull while my shields are recovering, this number just keeps going up, and all the rest of the time they are chewing through my 4000hp hull with 45% resists (and I'm being extra evasive while shields are down of course). So, 5625 effective health for the hybrid in a 10m engagement that results in death at the end of the 10th minute.
Since on a shield build you don't have the HRPs, you've got the shields, the cells and probably some mission equipment (cos why not, that's the advatage of shield builds, more space!), and that's it. When they're gone, your hull is paper. I haven't worked it out, but I'd be surprised if you can get 6000 effective health from a full shield build on an FAS, and even if you could, you've then got heat management of the cells and people like me who will NEVER let you get even one shield cell off, I'll just cancel them with feedback rails as soon as you push the button.
THat's why hybrid is king, as long as you have the space for the requisite (at least 4) HRPs and at least one (if only one, make it a size 5) MRP, but ideally two, a big one and a small one for the extra hardpoint resistance, unless you go sturdy on all weapons.
