i looked into why the editing keys take precedence over the repl bindings, but i'm lacking the knowledge of how keymaps and minor/major modes interact.
the problem is most probably around defun slime-repl-mode in slime-repl.el.
is has a (use-local-map slime-repl-mode-map), but still somehow the slime-editing-mode gets precedence.
fuzzy completion had a similar issue which i've resolved with the following code snipplet:
(setf minor-mode-map-alist (stable-sort minor-mode-map-alist (lambda (a b) (eq a 'slime-fuzzy-target-buffer-completions-mode)) :key #'car))
...but slime-repl-mode is not a minor mode, so this trick can not help there.
thoughts?