The way I see it is Horner has to be proved to have doped before I will be sceptical (edit might have better said "very sceptical"). There are arguments that can explain his performance for me.
1) He is a seasoned pro who knows his body and just as long distance runners in the 40's can steadily put in high quality performances, so might Horner. Over the past few years he regularly puts in top 20 Grand Tour performances (unlike Cobo).
2) The drug testing is better than ever - I really think they will find out the majority of people
3) We say his performance looks dodgy but I don't see anyone looking at data (which can be interpreted in various ways of course) the important thing is
4) This is the only Grand tour he has done this year. Why is it not possible for him to have nailed his preparation for this year and that to have put him physically on a par with younger guys at the very end of this race. 3 weeks of racing is about endurance - old guys can have very high capacity in this regard - it is not the explosive first week that he has been outstanding as such - he just seems to be a steady performer while other levels of performance drop off. Take a look at his competitors in the top 10. 4 of them did the Giro, 4 did the Tour and the other is Konig who is breaking through as a GC rider.
ITA 1 NIBALI, Vincenzo Giro
USA 2 HORNER, Christopher
ESP 3 VALVERDE BELMONTE, Alejandro Tour de France
ESP 4 RODRIGUEZ OLIVER, Joaquin Tour de France
IRL 5 ROCHE, Nicolas Tour de France
ITA 6 POZZOVIVO, Domenico Giro
FRA 7 PINOT, Thibaut Tour de France
CZE 8 KONIG, Leopold
ESP 9 SANCHEZ GONZALEZ, Samuel Giro
EST 10 KANGERT, Tanel Giro
We have to judge him against this and not the past. Bike racing has changed.
To say his performance is not normal is palpably true for the Vuelta: we maybe only have last year of Contador's comeback as a comparable "clean" race for comparison so nobody really knows what normal is any more and we ought to be open to the possibility that this is it.