No Dave
I think Steyn did not show enough in any time he was on the field long enough and aside from one line break - he was poor in defense, They only take three centers to the WC and the chances of Steyn's selection in that position is near to zero,
As to Mostert the line outs were called by him in the AB test and it was not effective since the line out formation was unstable. Mostert was outjumped in the first line out and took two other line-outs later in the game, What happened was also a lack of accuracy in line out throw-ins by the Springbok hookers as a result of instability,
In the Argentine test Mostert -
* always took up the line-out position of a 7 (second from the back) on throw-ins by the Springboks;
* was moved from the middle of the line-outs with Du Toit being either in the middle of the line-outs or in front moving Etzeneth to the middle of the line-outs from where driving mauls originated; and
* did not make the line-out calls at all.
It must be mentioned that Mostert was on test level always totally useless in forming and managing of driving mauls. the reason is his deficient physicality, In the Aussie test De Jager forced his way through a maul situation to score a try, When Mostert tried the same thing in the Argentine test he was forced back by the Argentine forwards.
Aside from that, in the Argentine test despite Mozart's distortion Mostert missed a tackle on Sanchez when the latter ran through a feeble tackling attempt. Aside from that miss - Mostert also missed four other tackles in the Argentine test,
He gave away a penalty straight in front of the goalposts that was converted by Sanchez and in the second half gave away another penalty, The penalty was blamed on Etzebeth - but in his clear comments the referee said 5 went off-side first and Etzebeth was blamed for the subsequent bringing down of the Argentine maul. So effectively Mostert gave away two penalties as well.
The above is enough to ensure that Mostert will not be in the WC side, Meyer made a total balls-up in the selection of the 2015 WC squad and took 8 passengers along which was basically unplayable in the series, That figure went up to 10 when De Villiers got injured and Matfield flopped whenever he played other than the test against the Argentine "B" side, Erasmus is not a fool - he knows the implications of taking along unplayable passengers and will not repeat the Meyer stupidity, So no places for Mostert or Steyn in the squad.