okay, your math is sound and I did overlook devourer because i'm not used to including jungle items in builds
so it depends upon how you would rather build her, if you build optimal AP magic damage
devourer, 1 zhonya, 1 void staff, 1 abyssal scepter(depending upon if you would prefer the magic pen or not, you could change this to Nashor's) 1 rabadon's and boots of choice, probably magic pen so you can have Nashor's for the offensive stats
it comes to (depending upon if riot changes zhonya's to 100 ap or not) 100/120 + 70 + 60 + 120 )) * 1.4 from rabadon's and masteries, this comes to 478 ap for optimal magic penetration or 646 if you sacrifice boots for another rabadons or etc, the numbers vary so keep 478 in mind
which closely resembles the figures you provided
if you choose optimal AP Values(which is probably the least effective depending upon if you keep boots or not(though boots are an absolute for most if not all players), the maximum ap you could achieve with devourers and boots is dependent upon the function of the build, for instance, with pure AP, (4 rabadons, since it will likely be the only 120 ap item left after the changes), comes to 672 or 840 if you sacrifice boots
478 * ( 0.50AP(with masteries and nashors) * 3) = 717 + 345(nashor's and elise's passive and devour's magic damage ) + 1.0(478 AP)(spiders)(again, i'm calculating for elise's total damage) +40( x 5 spiders * two hits = (400) + 200 AD = 2140 total damage and another 120 free damage from her innate magic damage on hit for 2260, 2060 of which is magic damage and that's before the steroids for just two hits, now if you add .9 AP from the additional 30% * 3 on hit procs, you have 2690 (or 2470 in magic damage)) before she even casts her spell as the finisher
if you take the maximum AP approach not including nashor's or boots for a total of 840
840 AP * .9(3 procs of passive) + 840(spiderlings) + 400(spiderlings) + 180 (devourer magic damage) + 120 (elise's passive magic damage) + 126 (5% mastery 3x) + 200 AD = 2506 total damage
now calculating the doubled ap ratio on her passive because of the active, it comes to 3378 damage before reductions
i'll list the 300 ap including nashor's and devourer as well
300 * 1.8 + 300(spiderlings) + 400(spiderlings) + 135(nashor's ap ratio) + 180 (devourer magic damage) + 120(elise's passive base magic damage) + 45(5% mastery 3x), + 45 (nashor's base 15 damage) + 200 AD = 1965 damage before reductions before spells
in short, it is a tremendous amount of free damage regardless of her build and each build would have its own advantages and drawbacks depending upon what the player wanted and of course their team composition with Elise
there may be other junglers/laners that benefit more from the hit effects(such as master yi, who many have mentioned and frequently builds glass cannon)
but this item seems to target many of the underused junglers and buff them to ungodly levels of damage once they actually obtain the item, which I think should be somewhere around 25 minutes into the game if they build the jungle item as their first completed item
also, I wish riot would reformat the way the forum posts are stacked