The gearbox may take a little getting used to, but it is worth it. As has been mentioned, there is a bit of a lag sometimes when pulling away from a standing start, but that's not a
problem overrall for me. The gear changes are so smooth and rapid, that's a bit of a mask on the speed sensation, but the results are there on the speedo. With a tuning box, as Tell said, power was great , but not as smooth as the remap. I've done about 1000 miles remapped now and can happily report no issues at all. If anything its 1 or 2 mpg better on fuel.
Not driven a Leon Cupra Mk1 TDi, but they are fast, no doubt, also a lot lighter and the engine characteristics are different as well. Handling on the Altea should be better by all accounts.
I have used a Toledo mk1 20vt just before getting my Altea and handling on the Altea is much better, performance about the same ,or remapped, just a bit quicker.
At the end of the day its a subjective thing, if you like it when you test it , then it works for you. Others like the cars they have and thats fine. There are certainly plenty out there to choose from!