.... Hi,
.. I too have a WagonR Lxi.. since more than 3.5 years now...
I don't think there is any difference in new and old wagonRs... the only thing being the maruti website.. that now shows 64bhp.. at my time it was 62!!!
First the seats...
You are right the front seats are lower than the rear, the rear are about 3-5 inches higher. They are made on a higher floor altogether... I think its the same for the Santro... (I don't know y? though!!)
Rear legroom... true, not much... but the main cause for that is the too much flexibility of the front seats..... e.g. Santro's seats go just halfway as back as wagonRs. .. that said there's good front legroom!!
Now acceleration .....
When we first bought WagonR... I was a .. kind-of fresh driver. And I also felt that its slower than our 800DX (5 gear).... BUT I always knew the Autocar figure.... of 14.5secs .. 0-100km/h.... and just for comparison the same for HM Lancer.. is 13.xx.....
Since the last 2+ years I have been exclusively driving it.... ... and it is FAST.. and FAST like ANYTHING!!!
When you get used to the car.. then try this!!!
First gear .... .. .. push the accelerator hard (no need to floor it).... and you won't even know when the car reached 35+. (its so quick compared to a 3 cylinder car that.... first time u try in traffic.... the 800 standing by your side would be in your rear view mirror

)
Second Gear....... It goes to 60.... as smooth as silk!!!
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Fifth..... ... u do 110km/h..... as if you are just crawling..... but after 115... u start to feel the effort..... and.. it would take you to 140+... without any fuzz.... but with a lot of sound.
... Just try to take care of a few things.....
Try to use.. a higher Octane fuel...... Speed 93....or Xtra Premium.
Use at least "Castrol GTX Magnatec"..... .. don't use normal servo or anything that Maruti puts!!... u'll feel the difference...... (Magnatec is hardly 100 or 200 rupees costlier than the Normal).
.... One thing I have to agree..... I also love the look of this car/wagon.... not just because I own it.... but since the very first day I saw it!!