ive tried moving off quickly in my TDi in 1st gear and it can be uncomfortable, you hit full rpm by 35 i think and the traction control is in there and you feel every change in power as the turbo kicks in, not so smooth (unless i dont have the knack)
I like getting to 10-15mph in 1st, going into 2nd then applying full power... feels a bit like a plane taking off!
If you go faster in 1st so that there are higher revs when you go into 2nd, the power will be there quicker as there will be boost pressure there immediately, but i kinda like the feel as it builds from 1200rpm upwards, the big push comes in around 17-1800.
To compare like with like Go to the likes of Parkers for the Weight and Power data and compare power to weight ratios, for example:
A leon TDI FR is 167hp and 1415kg, or 118.02HP/ton (=167/1.415)
A Cupra is 236hp and 1375kg, or 171.62HP/ton (=236/1.375)
This makes the cupra 53.66HP/ton more powerful than the TDI FR, or 45% more powerful.
You can use this for equivalents too, for a TDi FR with the extra weight to be as powerful as the cupra, you need 243HP (=171.62 x 1.415).
It may feel more powerful, and may do a range (such as 30-50, 40-60, etc) faster than the petrol both due to the torque. On sprint where the TDi needs a gear change, the petrol has the edge.
The aerodynamics and ancillary loads will also affect things. An older car wont be so streamlined but may also have a smaller or larger frontal area.
The older car may not have an AC unit taking several HP... Although, during high acceleration, many cars switch off the AC compressor. BMW even switch off the alternator thus putting every drop of engine power into the drive.
Stuart