Ishant has been disappointing but will deliver the goods in the long run I feel. We forget that he is just 21. RP is also just 23/24.
His speed will not magically reappear. It would seem that his 140kph pace came as a result of excitement and confidence for playing for India, something which eventually fades away. We often see bowlers debut with high speeds and then fade into fast medium, although probably not to his extent. The obvious examples are Munaf, Finn (just recently bowling at 90mph, bowls nowhere near that domestically), Siddle (not quite reached the heights of his early matches), Aamer (contentious one), Sidebottom, Bollinger, all who have shown themselves to bowl at 110% speed on their debut few games. An issue with Ishant is that he is currently bowling far slower than he was domestically prior to his debut, which is a worry. To me, I think he must get stronger. I'm not saying that he should bulk up, but it does not take a rocket scientist (not that one would help) to tell that someone as skinny as he is, couldn't do with putting on a bit of strength and lean muscle. He is 21, but at 130kph, with his accuracy, he won't trouble anyone for a long period of time.
With RP Singh, the speed is once again worrying. It is more straightforward with him though. He debuted as a 130kph bowler, put on some speed after some strength work at Leicestershire which eventually wore off after a few years. He was never the most accurate seamer either and one who seldom swings it and only in helpful conditions.
If Sreesanth is good though, he would WALK ito the team. He is the most talented among the current stock of bowlers but the discipline issues have always been there. So he is a dark horse but I wouldn't bank upon him.
Sreesanth is only handy when he bowls at 140kph and gets it to swing late. At 130kph, he is a joke. However, when he gets the speeds up, he can be a potent wicket taker, this only tends to happen when he has been playing cricket for a while without injury but he did seem to be showing signs of it in the last game. The last time I recall him bowling truly quickly was
here where he was clearly expensive but took plenty of wickets.
I wouldn't give up on Tyagi or VRV Singh yet. I think they have a future, just not in the near future.
Tyagi definitely needs work on his run up. As for VRV Singh, I cannot remember the last time he played a game, reckon he is seriously injured.