I still have doubts about Petrov. I think NH flattered him and now he's got a mid season replacent (the like who have struggled since the testing ban)
As for senna here is a list of mid season replacements since 09
Alguersuari - Pretty rubbish 09. improves massively when given two full seasons
Badoer- Yeah...
Grosjean- Slow and accident prone
Liuzzi- Remarkable first race at monza, slipped back after
Kobayashi- Very good. Top marks, but in raw pace clearly slower than Trulli and the 09 Toyota was no slouch.
Fisichella- Just a team swap and he struggled badly
Heidfeld- Unremarkable, but not terrible. Beats Kobayshi once in the final race. More experience than others on this list and has been Pirelli tester.
Yamamoto- Slow, Lapped by Senna
Klien- Looked solid, but i think the "blowing Senna away" was circumstantial and Abu Dhabi was a truer picture
Chandhok- Very Slow, Very accident prone in his one race. Not a shock he was looked over for India
Ricciardo- average at first but is improving to show real potential. Poor in India.
Senna- Great at Spa, Singapore and Monza, Struggles elsewhere.
Of that list id place Senna very near if not on top. Ricciardo and Kobayshi have perhaps done better, but there is question over Ricciardo teammate and he was beaten by NK in the last race, where as Kobayshi we only had two races to look at him, Senna looks as good as KK if we only count the first two races. Then he had a pre season at Sauber.
Based on that id like to see him get a chance whith proper testing under his belt. It would appear however that Renault look set to be impatient again.