Posted: 05/01/2013 at 06:37
Sometimes the projected height given is the height expected 'after 10 years' or some other stated period of time. You have to look in the small print on the website or in the book. The RHS website says 8-12 metres for gleditsia after a period of 20 - 50 years - so much depends upon growing conditions.
What about one of the eucalyptus cultivars? Although they will grow into big trees, they take very well to being coppiced or pollarded and if this is done they retain the attractive immature leaves, rather than develop the mature ones, and you can control the height. They also grow quite quickly so it wouldn't take too long before it replaces your gleditsia visually.