the gain will be pretty appreciable, perhaps 15 HP. The biggest part of this gain will be attributable to the replacement of the smog-control exhaust manifold, which has air injection tubes running up into the exhaust ports, with a header (and presumably a new 2.5' performance exhaust system). The flat-top "bad" carbs actually flow as well or better than the older SU's; they are "bad " because they are such a PITA to keep in tune as they age, and they have tons of gadgets on them that make them very complicated. So if the carb switch helps it's not because the round tops are better, but because the flat tops were not running right. And remember that the intake manifold from the 260 probably flows better than the older one with the SU's, so it's a good idea to use the newer manifold (and polish the runners while it's apart) even though you are using the old carbs.
Finally, you can tweak a lil more power out of the new setup by replacing the 260 dizzy with a single breaker electronic dizzy. use a dizzy from a 280z or zx, along with the appropriate module or GM substitue, or run one of the electronic conversions like pertronix. The smog control setup on the 260 included tuning it with too little advance (to help control nitrate emissions) and you can bump the advance up quite a bit (experiment until you start to ping) and get some more ooomph. .4 mm less deck height will boost compression but that by itself will be an imperceptible improvement. But the total proper replacement and tuning of the smogless setup will indeed make a noticeable change in power and is definitely worth the expense.

 

Thanks!