Is there anything suggesting that vampires are more powerful in lore? Abilities such as superhuman strengh, speed, regeneration, defying gravity etc...

Vampire abilities/powers vary depending on the work of fiction in which they are depicted, however, it would make sense if lore-wise they had more power/strength but were depowered in the actual games to avoid balance issues.

http://www.imperial-library.info/content/immortal-blood should answer a number of questions of what different vampires may be capable of. Note that the nature of the Volkihar is a subject of debate around these parts.

In Dawnguard we see Serana pick up a fully grown male elf and hold him above her head for a time with ease. She's not particularly well-built so I assume that vampires have superhuman (well, supernatural, even a novice mage should be able to achieve this easily with a fortify strength spell) strength. The strength of someone usually effects their speed, since their legs would have the superhuman infusemant that the arms have, I expect that they'd also have abnormal speed too.

Defying gravity seems to kick-in with Vampire Lords, their wings are far too small to lift a body of that size without magical aid. Though again, that can also be achieved without vampirism with some intermediate-complexity spells (levitation spells).

Any unusual regeneration is something I know nothing about, though again, that's simple Restoration magic to do that. Basically, any decent mage should be able to match or even surpass the innate abilities that vampires gain.