Hmm, I also have a TPS deck with 10 proxies but i playtested and ran a Cruel Bargain, which is a alternate print of Infernal Contract from the Portal set, and I too found it better than Windfall. At some point i even had Cruel tutor in place of a single Grim tutor! Didn't work out like i thought

I eventually cut Cruel Bargain/tutor from my list in place for Thirst for knowledge/Intuition. Both can be pitched to Force of will when Cruel tutor/C.Bargain/I.Contract can't. Another alternative would be to go with Gush, though it would depend on your meta. You could loose big time tempo using Gush against all of the Stax/MUD decks out there. Again, Gush also pitches to Force of will. Currently I am working on a non-proxie TPS and although many have advised me to go the Ad Nauseam combo route, I refuse because TPS and ANT play so much differently. I enjoy both equally but I enjoy not having the pressure of a turn one win and I am most familliar with most of TPS lines of play. Here's my deck list, hope it helps:
main deck:
4 polluted delta
1 bloodstained mire
1 Tolarian Academy
2 island
2 swamp
3 underground sea
4 chrome mox
1 lotus petal
1 mana crypt
1 lion's eye diamond
1 mana vault
1 sol ring
1 sensei's divining top
1 memory jar
1 darksteel colossus
1 gush
1 thirst for knowledge
1 intuition
1 ponder
1 brainstorm
1 fact or fiction
1 gifts ungiven
1 merchant scroll
1 mystical tutor
1 tinker
1 mind's desire
1 misdirection
1 chain of vapor
1 echoing truth
4 force of will
2 cabal ritual
4 dark ritual
1 demonic tutor
4 duress
2 tendrils of agony
1 vampiric tutor
1 yawgmoth's will
1 necropotence
1 yawgmoth's bargain
sideboard:
2 tormod's crypt
2 pithing needle
1 mana web
1 damping matrix
1 engineered explosives
1 sundering titan
2 hurkyl's recall
1 darkblast
1 massacre
3 yixlid jailer
The sideboard is still not how i would like it though, i am constantly tweeking here and there.
I wanted slaughter pact in place of Darkblast but i like taking care of all of the Gorillas,BOBs, and Welders in the late game as opposed to the one time use i get from slaughter pact. Again, this is my non-proxy decklist and i don't own P9. So far this is what i have. If i had my proxies in it, I would definately add a Seal instead of grim tutor and keep thirst for knowledge or gush for draw. intuition works for me, give it a try and let me know what you think. Also, I would leave out the timetwister or any other draw7 for that matter in the current meta with all the mana drain.decks around...