I actually use a very similar build when I play singed, the only difference being I prefer 
over Randuin's most of the time.
It does have a very noticeable mid-game power trough as your items ramp up and a big flaw it always has is that it has low MR, so it isn't great vs AP heavy teams. Usually, to minimize the point in the game where your damage and durability is lowest, I start catalyst into RoA instantly since you can stack tear so insanely fast on Singed that getting a completely stacked RoA is actually more important (since it has a set timer of 10 minutes, where you can stack tear faster than any other champ). After that, you'll want to get some resists ASAP, usually getting your merc treads and then a Warden's mail, at which point you can get either Rylai's for the slow (very strong option if you need to kite better or need more CC in team fights) haunting guise if you are ahead and want to start killing people (not the best unless you already have a lead) FH/Randuin's if you need to not die to an ADC or enemy fighter, or if you are pretty much ruling the roost, finish Seraphs early for a major boost in damage and durability (although it leaves your very squishy to things like BotRK)
This build has really big pay-offs late game with its insane DPS from your poison, but the risk will always be mid game being a bit tricky and you being vulnerable to % health damage (you'll usually end with something like 4500 health, but only about 150-200 armor and maybe 70-100 MR) It is a very high damage AP fighter-tank build, and not the best if your team needs a dedicated frontline, since you'll be diving behind enemy lines for their carries most of the time.
EDIT: In a splitpush situation, this build is fantastic since they won't ever catch you between the Rylai's slow, your ult, your ghost, and the fact that they'll be losing several hundred HP per second later on. You don't need to proxy farm in their base as singed (it can be an effective play style, but it isn't Singed's only play style) just proxy farming ahead of one tower is enough since mid-late game you don't want slightly higher global pressure that comes from base proxying, but rather directed pressure by forcing a wave against a tower and then battering it down before proxying the next wave.
with 9/21/0 masteries, flat m.pen reds, scaling armor yellows, scaling m.resist blues and M.speed quints.
I really like the sinergy between tear and R.O.A. but sometimes I feel really week mid-game with these builds so we lose objectives because of that.
