The impact tossed out about 500 billion tons of debris into the atmosphere, some of which escaped into space.
The difference in altitude from the perspective of the meteor would be utterly irrelevant. It's like suggesting there's a difference between stepping on a hardwood floor, and a piece of paper on the hardwood floor. The same 500B tons of debris would go into the atmosphere — but it is 4.5 Km higher off the sea floor, which means there is a difference in how the debris would be distributed initially in the atmosphere. Frankly, it would probably distribute faster due to lower gas density. But that's a bit of a guess on my part.
Conclusion: No difference.
The atmospheric temperature rose to the extent of provoking multiple spontaneous combustion, merging together into global forest fires.
Same force, but at 4.5 Km altitude the air is thinner. Would it have been thinner enough to make a difference? To reduce the compression wave that wove around the world?
500B tons of mass thrown into the sky....
Conclusion: No difference.
The impact melted the surrounding rock.
This is perhaps the most important observation. If this site is to be believed, the Chicxulub impact was all ocean. No land. The maximum Gulf of Mexico depths today (ignoring all the disputes) are about 13,000 feet deep. Even if we assume half that (1.25 miles or 2 Km), it's a whole lot of water to boil away to get to that 500B tons of material thrown into the sky. And yet the surrounding rock is known to have melted.
Have you ever seen a stone thrown into a hot mudpit? It doesn't leave a crater. In fact, for a moment, the area the stone entered is higher than the surrounding mud pit. There are a lot of reasons for this effect (not the least of which is the extra mass represented by the stone), but the result — if you could freeze it — is a plateau. Which is actually what we got. Compared to the depths of the Gulf of Mexico, the Yucatan Peninsula exists and the ocean around Chicxulub is shallow.
When you get to that level of force, the thin layer (2 Km at least) of water is almost if not completely meaningless. So, yeah, molten rock.
Conclusion: No difference.
The asteroid couldn't have landed at a worse place on Earth. The Gulf of Mexico was rich in sulfur and carbon, which many scientists believed compounded and even prolonged the fall of the dinosaur empire. From the sulfur, the first few years of the catastrophe saw a five-degree-Celsius drop in temperature before spiking up to a 20-degree rise that persisted for millennia, thanks to the carbon.
This, on the other hand, may be the most debatable condition. Carbon is a part of everything on this planet. The idea that there was more of it 65M years ago at Chicxulub than elsewhere on the planet sounds frankly daft to me. 500B tons of mass were thrown up in the air. The volumes we're talking about were astronomical. So, unless someone can prove Chicxulub 65M years ago was all diamond (500B tons of it), the amount of carbon was irrelevant.
The sulfur might be an issue. But the rock melted. The forests burned world-wide. 500B tons of mass. All of which might be irrelevant because there's a giant sulfur deposit on the Eastern Tibetan Plateau.
Conclusion: No difference.
The impact left behind a crater 93 miles wide and 12 deep.
This issue might actually be irrelevant.
Conclusion: I don't believe there would have been a significant difference in the final timeline. Oh, maybe something other than the alligators would have survived and the alligators themselves wouldn't, but when considering the timeline as a whole: no, nothing would have changed.