you can play mutagenic growth in any colored deck because of the phyrexian mana. It can serve two purposes: push in those last point of infect damage, or take your guy out of burn range/flash blockers/kill a hard to deal with blocker fast.
But if you want to keep your deck infect control I would suggest you take out the Blightsteel Colossus from the sideboard, you wont have time to cast it. Then decide if you want a proliferate theme deck or not, if so add more proliferate + mass removal (like black sun's zenith).
But I suggest you do the following and see if you like the deck:
SB:
out Blightsteel Colossus
out 1-2 Reaper of Sheoldred (4 is too mutch)
out myr
less think twice (max 2 or 0)
move contagion engine to the side,
take out (or move to side)Inexorable Tide, cause if you play this and they play a titan, you are probably dead. Move one Tezzeret's Gambit to the sideboard. Change a Hand of the Praetors to a Skithiryx, the Blight Dragon. Also consider taking out 1-2 Cryptoplasm for extra land. Change one or two Corrupted Resolve with mana leak/negate for consistency. Also because of the drawback of doom blade, change one to Go for the Throat.
the Thrummingbird is a waste of space, because it wont deal infect and the proliferate is not that significant (note if you want a proliferate deck, make one.. this is not) also it does not combo well with anything (not the dager, it will just do 3 damage and proliferate once, neither with the engine, use tezz's gambit). Instead of them add 2 bounce spells likeVapor Snag, Silent Departure (can be used twice) or Disperse. Disperse goes well against enemy plainswalkers or equipment. Also all the bounce spells can remove a pesky blocker for a turn or buy a turn against a titan so you can counter it. Also copying something with the Cryptoplasm than bouncing the original (like a titan) than attack, this can win you the game on its own.
Also consider Phantasmal Image instead of the cryptoplasm, it is easier to cast and enters the battlefield as the copy, you don't have to wait a turn to make it.
Add some (2)Black Sun's Zenith to the side or Tribute to Hunger against the hexproof guys.
This will reduce the deck to a normal 60 card deck and make it more consistent.