I did the maths, using the numbers on the soasewiki but there are some question marks...
I assume approx 3,5 credits per unit crystal or metal
all ships regen shield at 1/s and hull at 0,33/s during combat, mitigation subtracts a further 15% from this, I have chosen not to count with increased mitigation since damage under 12,5 DPS doesn't increase mitigation
I assume that antilight gets 200% against LRM shields and armour making 6 DPS for the seeker, after regen that makes it 5 DPS against shield and 4,66 DPS against armour
I assume each point armour subtracts 5% total damage
I assume antimedium gets 100% against seekers making 11 DPS for the LRM and 13 DPS for the Kanrak respectively, down below are regen corrected values
LRM does 10 DPS against shields and 9.66 against armour
Kanrak does 12 DPS against shields and 11.66 against armour
hp = hitpoints (shield + hull(armour included)) bt = build time s = supply
Javelis LRM cost 275 $ 45 M 25 C approx 520$ hp 805 (280 + 525) bt 21 s 4
Kanrak cost 400 $ 65 M 45 C approx 785$ hp 1020 (360 + 660) bt 23 s 6
Seeker cost 200$ hp 865 (425 + 440) bt 13 s 2
Ship firing at Ship, time for shields to drop, time for hull to explode, time to die
1 seek vs 1 Jav 280/5 = 56 s 525/4,66 = 112,66 s 168,66 s
2,6 seek vs 1 Jav 280/13 = 21,5 s 525/12.12 = 43,33 s 64,83 s
1 jav vs 1 seek 425/10 = 42,5 s 440/9,66 = 45,54 s 88,04 s
1 seek vs 1 kanr 360/5 = 72 s 660/4,66 = 141,63 s 213,63 s
3,9 seek vs 1 kanr 360/19,5 = 18,46 s 660/18,17 = 36,32 s 54,78 s
1 kanr vs 1 seek 425/12 = 35,42 s 440/11,66 = 37.73 s 73,15 s
so from this table it can be surmised that a single seeker never stands a chance but given that is is cheaper than both LRMs we can assume a numerical superiority, resource equivalence seems to be 2,6 seekers per Javelis and 3,9 per every Kanrak.
Break even damagewise (where you expect 1 seeker to pop for every LRM) seems to be 1,9 seekers per Javelis and 2.9 seekers per kanrak
Not only do the numbers add up

you end with a financial net gain.
So to sum it up, seekers seem to be a viable LRM spam counter, but they are severely lacking against almost everything else (useful against siege, colony and constructors for whatever it is worth...)
Let the seeker spam begin!