oh yeah. i presumed C=C means sp2 hybridised, ie. I erred. you are right that it remains sp.
Regarding the formation of allene from alkyne, it would means that 1C=C and 1 C=C is converted to 2C=C. I included the average bond energy below:
C=C = 820 kJ/mol (196 kcal/mol), C=C = 611 kJ/mol (146 kcal/mol), C-C = 368 kJ/mol (88 kcal/mol)
dH = +(820+368) - 2(611) = -34kJ/mol
dH is negative to break 1C=C and 1C-C to yield 2 C=C. Rearrangement to form C=C would be energetically favourble.