Slip and Turn
I'm hoping that you realise 2 things
1) That V1 is NOT a fixed speed. It is dependant on a whole host of factors such as weight, runway state, runway length, flap setting and so forth.
2) Engine thrust is NOT a fixed setting. You can reduce thrust if you do not need all of it to save engine life. Hence if you are heavy on a short runway you use a much greater percentage of your maximum available thrust than if you are light on a long runway. However the net overall effect is the same. Put in very simple laymans terms you trade thrust for runway length depending on weight.
I really reccomend that you do some research on the balanced field concept. It is the crux of modern performance calculations. Granted it's not an easy concept to understand if explained badly, but all perf calcs are based on balanced field in one way or another.
I honestly think you have a basic misunderstanding of performance A calculations.
I will always do performance calculations based on the most limiting intersection i am likely to get. If i'm too heavy i'll take a less limiting intersection. If ATC need me to take full length for whatever reason i'll take full length. However i could take the intersection if i needed/wanted to without eroding any safety factors.
Also from your messages i believe you're talking about ryanair. I do not work for them, however i am lead to believe that their SOP's are second to none, and they will not allow anything to compromise safety.