About a second to 60, and about 8mph top speed, depending on who you believe.
Engine, trim, body trim, wheels, brakes, suspension, interior etc all different.
Getting to 150BHP from a standard 2.0 GTi, which is 137, is not an easy task, simply because the engine doesen't have much more to give. Some headwork, cams, natched inlet and exhaust and a remap may do it, but will cost more than chopping it in against a 180 anyway. A remap alone won't get you 12BHP I'm afraid, though Superchips claim they can give 11 (I'm a little suspicious of that figure).
The standard GTi is to my mind very underrated, with a good strong midrange. I'd give it a top-to-bottom service with top quality lubricants, iridium plugs, and maybe a panel filter. I'd then beef up the front brakes with new standard (or Brembo) discs and the performance pads of your choice, some quality rubber and you'll have a sweeeeeeet little machine with nothing to upset the insurers
Superchips reckon it's not safe to go beyond another 10ish bHP without some other mods - the OBD management won't trim beyond its mapped fuel/air range, making it liable to run lean and cause damage at high loads if the remapping is taken beyond the extreme. It's already in a fairly high state of tune compared to the original base design, and going for big figures also loses the meaty mid range - you don't get more area under the graph, just different peaks. These guys know their stuff, having extracted 31 BHP from my own engine (although mine as standard is in a lazy state of tune) so I wouldn't disbelieve them. That said, several GTi's in the PSOOC made significantly beyond 180BHP at the wheels in a barometrically corrected dyno, so the 180 claim can be considered pretty fair.
In detail it's a very different engine from the 180 unit, and if it were that easy I'm sure pug would've done that for the 180 and saved themselves a heap of development costs. By the time one has fannied about getting a safe and flexible 150 BHP (a peaky and nasty 150 BHP probably isn't so hard) one could've bought a 180 anyway, and benefitted from the interior, exterior kit and alloys.
As aformentioned, I reckon they're great cars but people just see the headline figures of the 180 and don't give the standard GTi a chance, a bit like the old 1.6 205 GTi. The biggest performance gains for a sensible price are in the tyres and brakes. Good rubber, decent brakes, a tip-top motor fully serviced and on song make these a deceptively swift machine, but a nastily tuned motor on bolloxed tyres and shot brakes is as much fun as a night in the cells with Bubba.