может быть для отображения использовать метод paint твоего делегата?

void QmyDsbDelegate::paint (
        QPainter *painter, 
        const QStyleOptionViewItem& option, 
        const QModelIndex& index ) const {

    QString text;
    QRect rect;
    QVariant value;
    QStyleOptionViewItemV2 opt = setOptions(index, option);
    value = index.data(Qt::DisplayRole);
    text = QLocale().toString(value.toDouble(), 'f', 2);
    opt.displayAlignment = Qt::AlignRight | Qt::AlignVCenter;
    drawDisplay(painter, opt, opt.rect, text);

QString QLocale::toString ( double i, char f = 'g', int prec = 6 ) const
This is an overloaded member function, provided for convenience.
f and prec have the same meaning as in QString::number(double, char, int).

QString QString::arg ( double a, int fieldWidth = 0, char format = 'g', int precision = -1, const QChar & fillChar = QLatin1Char( ' ' ) ) const
This is an overloaded member function, provided for convenience.
Argument a is formatted according to the specified format, which is 'g' by default and can be any of the following:

Format                               Meaning
______________________________________________
e             format as [-]9.9e[+|-]999
E             format as [-]9.9E[+|-]999
f              format as [-]9.9
g            use e or f format, whichever is the most concise
G            use E or f format, whichever is the most concise

With 'e', 'E', and 'f', precision is the number of digits after the decimal point. With 'g' and 'G', precision is the maximum number of significant digits (trailing zeroes are omitted).
 double d = 12.34;
 QString str = QString("delta: %1").arg(d, 0, 'E', 3);
 // str == "delta: 1.234E+01"
The '%' can be followed by an 'L', in which case the sequence is replaced with a localized representation of a. The conversion uses the default locale, set by QLocale::setDefaultLocale(). If no default locale was specified, the "C" locale is used.