There is an efficiency difference - while a lot of the starch in caramel malts have been converted, there is still some remaining that can be extracted in a mash, but not in a steep. Also, the mash is typically done for longer than a steep, plus a sparge, which extracts more sugars from the grain. 30% extraction for a steep seems on the low side - but let's go with that for now.
In principle, if you were getting 30% extraction but now get 75%, then you would need to use 30/75 = 40% of the original 1lb of speciality malts to arrive at the same gravity. But doing that means you are going to lose a lot of color and flavor, that being the main contribution from the speciality malt. Instead, to preserve the color and flavor, keep to the 1lb of speciality malts and adjust the quantity of pale malt to account for the increased GUs from the speciality malts.
So, how many additional points from the speciality malt are you getting with the increased efficiency? Assuming your 1lb of speciality malt is a mainly mix of caramel malts, which have a typical yield of 35 points, then you'll now get 35*0.75 = ~26 ppg compared to 35*0.30 = 10.5 ppg from the original efficiency. Scaling that to a 5 gallon batch, that's about 5 points vs. 2 points, so you'll gain an additional 3 points with the increased efficiency in the mash.
To hit the same OG as the original recipe, you then reduce the amount of basemalt so that it produces 3 points less.
Brewing calculators, such as Beer Calculus make this process a lot simpler.
30% yield from steeping does seem a little low to me - 50% is probably a better estimate of typical steep efficiency across all malts. In practice it varies for each malt type.
For example, caramel 40L has a typical potential extract of 35 points. John Palmer reports that after steeping caramel 40L he gets 22 points, so an efficiency of 22/35 = 62%. That's the best efficiency reported for the malts tested and is because most of the starches are already successfully pre-converted during the special malting process used to produce these medium crystal malts. The efficiency of dark crystal and chocolate was lower - 45% and 42% respectively. Averaging all of these (taking typical quanities into account) gives about 50% which is often quoted as the typical extract from steeping.