I can understand the political reasons behind His execution. What I can't really understand is why did the Romans go through such a lengthy and gruesome process. I know crucifixion used to be a somewhat common penalty at the time but every time I read the accounts of it I am overcome with a sense of exaggeration, of unbounded excess. Was it so important for the rulers to make an example?
Crucifixion was somewhat common in that time, as your question points out. The issue I think you are hinting at is maybe Christ's beatings by the Sanhedrin and Romans before His crucifixion. The first is that the Sanhedrin heard Him blaspheme right in front of them and would have stoned him on the spot according to their laws, but it is apparent they were not allowed to execute. The other beatings occurred after he was handed over to the Romans. Pilate had Him beaten and lashed before handing Him over to the mob for execution. There is an old and common argument that Pilate did not want to execute Christ because he was under the impression that His followers may revolt, so instead he had Him beaten, hoping the punishment would be enough to appease the mob. Obviously it was not. So it might be possible that severe beating before being crucified was uncommon, so perhaps Christ did suffer more in this respect. However, according to Matthew, Jesus was flogged after Pilate agreed to hand Him over to the mob knowing full well what they were going to do to Him. Perhaps then a flogging before execution was common. Other beatings occurred when He was under the care of the cruel guards. They mocked him and put thorns on his head. I would think this kind of treatment was common, but I have no source.
After the mob took over Christ was apparently made to carry the very cross He was to be hung from all though the city. I have read in certain places that this may have been anywhere from one half to three miles. Considering if was under the jurisdiction of the Pharisees (mob) at this time I tend to think that this part of his suffering was uncommon. Even until recent times, typically the executed were disgraced after their execution by desecrating the body. Crucifixion in itself yields this result; the bodies were often left there long after death. What has also been common throughout the ages is dragging the body behind a horse or staking all or parts of it, like the head, in the middle of the town.
While on the cross it seems that Christ actually suffered less, relatively. The first point is that he died much more quickly than was typical. Crucifixion was designed to prolong suffering for days, and Christ passed within a half day. To hurry death they would break the legs so that asphyxiation would come quickly (there is kind of a long explanation here of the mechanics and the biology that is not in the scope of the question). But they did not break his legs.
Before sentenced to execution Christ suffered severe beatings, which is not really uncommon for the time, but he was beaten by several parties all wanting to exact their own justice on Him. One party wanted to stone Him but were not allowed, another was maybe hoping to save His life to appease the mob, and the other by cruel guards. He was sentenced to crucifixion anyway by the mob that also disgraced Him by making Him carry His own cross through the town. Considering He was already weak from the beatings this was probably a very difficult task. However, while on the cross it seems Christ's suffering was relatively less because he died in less than a day when it was typically days to die.
With that I would say He suffered more before the actual hanging because multiple parties 'wanted a piece of Him'. He suffered more still, being disgraced before He was even put on the cross. However, He suffered less on the cross because He lived less than a day and they did not break His legs.
Wikipedia on crucifixion covers much that supports what I have said above including prolonging death and desecration of the body after death.
Matthew 26 begins the story of Jesus' arrest (26:47), beatings (26:67), handing over to the mob and further beatings (27:26, 27:30) and crucifixion (27:35).